We have done something similar to what you are requesting, however we did not test it for Salesforce1. We dummied down the standard functionality in the following way. Related To picklist allows user to select Object to which Email is related. We removed this and automatically associate Email with Case which was all we required for our Use Case. We still supported Attachments - done slightly differently and also did not have lookup fields for Additional To, BCC, CC fields. We also set the From field - we do not want them to be able to update it themselves, but that can be changed if it does not meet your requirements. Please let me know if this is helpful.

Here is the basic VF page

<apex:page standardController="Case" extensions="SendCaseEmailController" action="{!populateTemplate}">
    <apex:sectionHeader title="Task" subtitle="Send an Email"/>

    <style>
        .boldText {
            font-weight:bold;
        }
        .labelText {
            font-weight:bold;
            display:inline-block;
            text-align:right;
            width:100px;
        }
        .wideTextArea {
            width:400px;
        }
        .mediumTextField {
            width:200px;
        }
        .longTextField {
            width:400px;
        }
        .extraDeepTextArea {
            width:550px;
            height:200px;
        }

    </style>
    <apex:form>
        <div style="display:none">
            <apex:inputField value='{!Case.Subject}'/>
        </div>
        <apex:pageBlock id="dataSection">
            <apex:pageMessages id="pageMessages" />
            <apex:pageBlockButtons >
                <apex:commandButton value="Send" action="{!send}"/>
                <apex:commandButton value="Cancel" action="{!cancel}" immediate="true" />
            </apex:pageBlockButtons>            
   
            <apex:pageBlockSection title="Edit Email">
                <apex:panelGrid columns="2" cellpadding="5%" cellspacing="5%">
                    <apex:outputLabel styleClass="labelText">To</apex:outputLabel>
                    <apex:inputText id="idRecipient" styleClass="longTextField" required="true"
                        value="{!emailMsg.ToAddress}"/>

                    <apex:outputLabel styleClass="labelText">From</apex:outputLabel>
                    <apex:outputLabel id="idSender" value="{!emailMsg.FromAddress}"
                        styleClass="longTextField"/>

                    <apex:outputLabel styleClass="labelText">Related To</apex:outputLabel>
                    <apex:outputField value="{!emailMsg.Subject}"/>

                    <apex:outputLabel styleClass="labelText">Additional To:</apex:outputLabel>
                    <apex:inputTextarea id="idAddlRecipients" styleClass="wideTextArea"                         value="{!addlRecipients}"/>

                    <apex:outputLabel styleClass="labelText">CC:</apex:outputLabel>
                    <apex:inputTextarea id="idCcRecipients" styleClass="wideTextArea"                       
                        value="{!emailMsg.CcAddress}"/>

                    <apex:outputLabel styleClass="labelText">BCC:</apex:outputLabel>
                    <apex:inputTextarea id="idBccRecipients" styleClass="wideTextArea"                      
                        value="{!emailMsg.BccAddress}"/>

                    <apex:outputLabel styleClass="labelText">Subject:</apex:outputLabel>
                    <apex:inputText id="idSubject" required="true" styleClass="longTextField" 
                        value="{!emailMsg.Subject}"/>

                    <apex:outputLabel styleClass="labelText">Attach File</apex:outputLabel>
                    <apex:inputFile value="{!attachment.body}" filename="{!attachment.name}"/>

                    <apex:outputLabel styleClass="labelText">Body:</apex:outputLabel>
                    <apex:inputTextarea id="idBody" styleClass="extraDeepTextArea" 
                        value="{!emailMsg.TextBody}"/>

                </apex:panelGrid>
            </apex:pageBlockSection>
        </apex:pageBlock>
    </apex:form>
</apex:page>

Here is the Controller 

/*
** Class:  SendCaseEmailController
** Created by XYZ on 03/20/2015
** Description: Controller for the SendCaseEmail custom Visual Force page. 
*/
public with sharing class SendCaseEmailController {

    public String addlRecipients {get; set;}
    public Case   ourCase {get; set;}
    public EmailMessage emailMsg {get; private set;}

    private OrgWideEmailAddress sender = null;

    private static final String SUPPORT_EMAIL_ADDRESS = 'support@somewhere.com';
    private static final String SUPPORT_NAME = 'Support Email';

    public SendCaseEmailController(ApexPages.StandardController controller) {
        ourCase = (Case)controller.getRecord();

        // create our EmailMessage 
        emailMsg = new EmailMessage();

        // get our org-wide email address to set from/sender field
        sender = [select Id from OrgWideEmailAddress where DisplayName = :SUPPORT_NAME];
    }

    public Attachment attachment {
        get {
            if (attachment==null) {
                System.debug('==========> creating new empty Attachment.');
                attachment = new Attachment();
            }
            return attachment;
        }
        set;
    }

    // send email message per the attributes specified by user.
    public PageReference send() {
        try {
            // now create our SingleEmailMessage to send out.
            Messaging.SingleEmailMessage singleEmailMsg = new Messaging.SingleEmailMessage();

            // concatenate all Bcc Addresses
            if (emailMsg.BccAddress != null && emailMsg.BccAddress != '') {
                singleEmailMsg.setBccAddresses(emailMsg.BccAddress.split(';'));
            }

            // concatenate all CC Addresses
            if (emailMsg.CcAddress != null && emailMsg.CcAddress != '') {
                singleEmailMsg.setCcAddresses(emailMsg.CcAddress.split(';'));
            }
            singleEmailMsg.setOrgWideEmailAddressId(sender.Id); 

            singleEmailMsg.setSubject(emailMsg.Subject);
            singleEmailMsg.setPlainTextBody(emailMsg.TextBody);

            // now add additional recipients
            String[] addlToAddresses = null;
            if (addlRecipients != null && addlRecipients != '') {
                addlToAddresses = addlRecipients.split(';');
            }
            // now lets add any additional recipients to our list of recipients.
            List<String> lstToAddresses = null;
            if (addlToAddresses != null) {
                // now append these to our main recipient.
                lstToAddresses = new List<String>(addlToAddresses);
            } else {
                lstToAddresses = new List<String>();
            }
            lstToAddresses.add(emailMsg.ToAddress);
            singleEmailMsg.setToAddresses(lstToAddresses); 

            // now we need to reset the ToAddress for our EmailMessage.
            emailMsg.ToAddress += (addlRecipients != null ? ';' + addlRecipients : '');

            // now attach file to email if there is one. Have to check the Body as Attachment
            // itself will never be null as it is always created first time it is accessed.
            if (attachment.Body != null) {
                Messaging.EmailFileAttachment emailAttachment = new Messaging.EmailFileAttachment();
                emailAttachment.setBody(attachment.Body);
                emailAttachment.setFileName(attachment.Name);
                singleEmailMsg.setFileAttachments(new List<Messaging.EmailFileAttachment> {emailAttachment});
            }
            List<Messaging.SendEmailResult> results =  Messaging.sendEmail(
                new List<Messaging.SingleEmailMessage> {singleEmailMsg});

            // now parse  our results
            // on success, return to calling page - Case view.
            if (results[0].success) {
                // now insert EmailMessage into database so it is associated with Case.
                insert emailMsg;
                // and insert attachment into database as well, associating it with our emailMessage
                if (attachment.Body != null) {
                    attachment.parentId=emailMsg.Id;
                    insert attachment;
                }

                PageReference pgRef = new PageReference('/' + ourCase.Id);
                pgRef.setRedirect(true);
                return pgRef;
            } else {
                // on failure, display error message on existing page so return null to return there.
                String errorMsg = 'Error sending Email Message. Details = ' + results.get(0).getErrors()[0].getMessage();
                System.debug('==========> ' + errorMsg);
                ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.ERROR, errorMsg));
                return null;
            }
        }
        catch (Exception e) {
            // on failure, display error message on existing page so return null to return there.
            String errorMsg = 'Exception thrown trying to send Email Message. Details = ' + e;
            System.debug('==========> ' + errorMsg);
            ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.ERROR, errorMsg));
            return null;
        }

        return null;
    }

    // cancel creation of emailMessage. 
    public PageReference cancel() {
        // no need to do anything - just return to calling page.
        PageReference pgRef = new PageReference('/' + ourCase.Id);
        pgRef.setRedirect(true);
        return pgRef;
    }

    // populate our Email Message Template from the database, filling merge fields with
    // values from our Case object. Then store resulting template in EmailMessage for
    // display to end-user who is free to edit it before sending.
    public PageReference populateTemplate() {
        // we need to perform the merge for this email template before displaying to end-user.

        EmailTemplate emailTemplate = [select Body, HtmlValue, Subject, DeveloperName, BrandTemplateId 
            from EmailTemplate where DeveloperName='EMAIL_TEMPLATE_NAME' limit 1];

        // construct dummy email to have Salesforce merge BrandTemplate (HTML letterhead) with our email
        Messaging.SingleEmailMessage dummyEmailMsg = new Messaging.SingleEmailMessage();
        dummyEmailMsg.setTemplateId(emailTemplate.Id);
        // This ensures that sending this email is not saved as an activity for the targetObjectId. 
        dummyEmailMsg.setSaveAsActivity(false);

        // send dummy email to populate HTML letterhead in our EmailMessage object's html body.
        String[] toAddresses = new String[]{YOUR_RECIPIENT_EMAIL_ADDRESS};
        dummyEmailMsg.setToAddresses(toAddresses);
        dummyEmailMsg.setReplyTo(SUPPORT_EMAIL_ADDRESS); 
        
        // now send email and then roll it back but invocation of sendEmail() 
        // means merge of letterhead & body is done

        // TargetObject is User. This tells the emailMsg to use the email message
        // associated with our dummy User. This is necessary so we can populate our
        // email message body & subject with merge fields from template
        Savepoint sp = Database.setSavepoint();

        Account dummyAcct = new Account(Name='dummy account');
        insert dummyAcct;
        
        Contact dummyContact        = new Contact(AccountId=dummyAcct.Id);
        dummyContact.FirstName      = 'First';
        dummyContact.LastName       = 'Last';
        dummyContact.Email          = 'nobody@nowhere.com';
        insert dummyContact;

        dummyEmailMsg.setTargetObjectId(dummyContact.Id);
        dummyEmailMsg.setWhatId(ourCase.Id);
        Messaging.sendEmail(new Messaging.SingleEmailMessage[] {dummyEmailMsg});
        // now rollback our changes.
        Database.rollback(sp);

        // now populate our fields with values from SingleEmailMessage.
        emailMsg.BccAddress  = UserInfo.getUserEmail();
        emailMsg.Subject     = dummyEmailMsg.getSubject();
        emailMsg.TextBody    = dummyEmailMsg.getPlainTextBody();
        emailMsg.ToAddress   = dummyEmailMsg.getToAddresses().get(0);
        emailMsg.FromAddress = SUPPORT_EMAIL_ADDRESS; 
        emailMsg.CcAddress   = '';
        emailMsg.ParentId    = ourCase.Id;
        return null;
    }
}