OK.
Point 1 is not to beat (at all).
If you like/need read/write access to set explicitly, there is no other chance than ext (1024 bs ?). The vFat image itself may be mounted write protected, but only the complete imgae (not down to single directories).
And I do not mind anyone to use ext, my personal favorite is vFat.

Point 2 may also be achieved with whatever file system you use.
)ust map the single Map Providers as storage files (e.g. OSMI and OSMII) to as many directories you need under /home/user/MyDocs/.maps/xxx.