Please suggest best way to accomplish below mentioned task
1. We want that when new Lead is created then its LeadOwner is assigned with the help of RoundRobin logic.
2. If LeadOwner is updated through salesforce user interface then it should be changed.
3. But when Lead is updated through data import then its LeadOwner should not change. Even if there is LeadOwnerId in imported CSV

Please suggest best way to accomplish below mentioned task
1. We want that when a new lead is created then its Lead Source is assigned in Lead object. 
2. But when its Lead Source is updated then in Lead object original Lead Source stays.
3. Updated Lead Source is stored in child object for history tracking.
4. Leads can be created/updated through data import wizard and its LeadSource can be changed.
5. Lead can be manually updated and its LeadSource can be changed.