Content which you buy from the Microsoft Store is a bit unclear though any ingame purchases made using AC may certainly transfer as part of the game's PC and Xbox One cross save.
As for cross region, we dont know if the game's planned to have a release in EU. If it does however, I dont think it would be a separate "version" but rather just having the game downloadable via the EU MS Store as well as being able to claim AC in the same manner. As a result it really wouldnt matter what account is used in the end...
(tho keep in mind that regions are not an account setting but a platform setting, you can use the same account to view other region's content by simply changing your Xbox One / Windows 10 region)