Hello,
I read your specs and basic requirements are clear.
Basically you want a feature/function in your admin which can download your existing database to your local PC and after editing, when you want to re-upload it, there should be a place in admin to upload your modified data.
Am I getting correct?
Still I'd like to discuss few more things, so if you can come to chat, that would be great!
I'LL UPDATE MY BID AFTER DISCUSSION
Regards,
Anurag.