As it seems there will be no support for attachments, images or tree-sync in org-mobile.el. Therefore I think the approach should to use git as a transport and fiddle in org-mobile.el later (if it's still necessary).
The best strategy I can think of is that every write from MobileOrg comes be in a separate branch if head is different from the last fetch.
That and a default setting wether MobileOrg or the repository should win if there are conflicts should be sufficient to avoid conflict handling on the mobile device.
I'm not sure if this approach is too naive. I would appreciate any thoughts on this.