Certain flash cards can use backups of saves. I use the EZF Advance, which has a built in clock so pokemon games will work. It can have roms and saves from ba uploaded onto it and downloaded off of it. Beware though, some don't work that way. I also have an XG2 turbo (not plus) which has an internalized memory stick that lets you backup save games onto the cart, but you can't transfer them to a computer or anything from a computer onto it. I would go for the EZF Advance cards as they're quite reliable, the save batteries don't die like the XG models and they're all around good cards. I have the first generation version of it and it's great (the EZF Advance is not to be confused with the EZ Flash Advance cards).
As for playing a us game vs an European game, I'd think it was possible, but don't take my word for it. I'm just saying it's quite possible that you can. all flash carts are capable of multiplayer support for GBA. Cept maybe the supercard, I don't really know.