One workaround was explained in this Dreamforce session "Using 3rd Party JS Libraries With Lightning Components (https://www.salesforce.com/video/303933/)".

• https://www.salesforce.com/video/303933/

They ended up creating a VF page containing the Leaflet map and embedding it in an IFRAME inside the Lightning component, then passing arguments to the IFRAME.

Another workaround is posted below:  delete and recreate the map on the accountSelected event.

Please note that I've declared an account list on a new component attribute 'accountList'
 

<aura:attribute name="accountList" type="List"/>

and then changed the accountsLoaded event to copy the accounts to it
 

accountsLoaded: function(component, event, helper) {
        // Add markers
        var map = component.get('v.map');
        var accounts = event.getParam('accounts');
        component.set( 'v.accountList', accounts );
        for (var i=0; i<accounts.length; i++) {
       ...

then changed the accountSelected event as below to recreate the map and restore the markers:
 

accountSelected: function(component, event, helper) {
        // workaround:  delete and recreate the map with the markers
        var map = component.get( 'v.map' );
        if( map ) {
              map.remove();
        }
        
        // get account coordinates
        var account1 = event.getParam('account');
        var latlng = [ account1.Location__Latitude__s, account1.Location__Longitude__s ];
        
        // initialize the Leaflet plugin passing the 'map' DIV
        var map = L.map( 'map'
                  , { zoomControl: false }
                  ).setView([37.784173, -122.401557], 14);
        
        L.tileLayer( 'https://server.arcgisonline.com/ArcGIS/rest/services/World_Street_Map/MapServer/tile/{z}/{y}/{x}'
                    , { attribution: 'Tiles © Esri' } 
                   ).addTo( map );
        
        var accounts = component.get( 'v.accountList' );
        for (var i=0; i<accounts.length; i++) {
            var account = accounts[i];
            var latLng = [account.Location__Latitude__s, account.Location__Longitude__s];
            L.marker(latLng, {account: account}).addTo(map).on('click', function(event) {
                helper.navigateToDetailsView(event.target.options.account.Id);
            });
        } 
        map.panTo([account1.Location__Latitude__s, account1.Location__Longitude__s]);
        
        component.set( "v.map", map );
            
}

There is still a glitch:  the map stops responding to the mouse drag and doesn't scroll.