Yes, that's right. If it can be operated offline without a connection to the server, the currency is not expected to revert back
no, it is not possible
you see, in terms of program engineering anykind of data either can be stored locally or... server. in gfl case, some data are stored and can be modified via files. but some, like anykind of currency they need confirmation from server's feedback
yes you can change the number "only on display" but the server said otherwise
as you asking is it possible to make it offline is not possible either, when you're offline the game try to reconnect by itself. the only way to play offline is, make your own private server (and that would be some nutjob who do that xD )