Can I continue to use my old Amazon UK account and use the European account for FR, DE only?

It is possible to continue to use your existing marketplace account(s) and the new European marketplace account without conflicting with each other. SellerExpress will use the NWS details to determine which Amazon account to interact with, as long as you don’t update your existing account details with the European accounts NWS details you’ll be able to continue to use both accounts.

Please be aware that Amazon will charge you for two accounts.