.3ds and .3dz created savegames are incompatible.
You can't keep the same save file when switching from EUR to USA, but you can use any ROM's region (just find some saves with the same progress you had which matches the new game's region).
Bravely default has online capability, but you need to use .3dz format (and if you find a save specifically for the .3dz format, don't forget that you need to inject the same Game Header string in your ROM or the save won't work).
Yes, it's possible to inject villagers into your save, I don't know if there are users willing to do it for you.
I made a lot of saves for Bravely Default EUR. (at least one per chapter).
You can find them here:
http://filetrip.net/3ds-downloads/game-saves/download-bravely-default-eurcyan-lv99-fin-f32852.html
look at the "other version (17)" in the menu on the right to find one with a progress you want.