Yes, you can use someone else's save file, and for that, I have the following guide in case if you're interested. This question remains unconfirmed, but I'd probably say no, because both versions store saves in a completely different way to the point that Steam version doesn't recognize save from MS Store version of the game, and vice versa.
No, you can't Jerry-rig, and instead you will be greeted with repeating "Invalid Profile" message until your game crashes it way out with error code "E:88-d"
Unfortunately, but you can't move over MS Store saves to Steam :( Can you move over MS Store save files to Steam version FH4?.