Future Cruise Deposit/Booking onboard

Is it possible to buy FCD's onboard and do you get the same benefit of extra OBC as if you had booked onboard . If you do book onboard are the rules still the same where you can either leave the booking with Cunard or transfer to the cruise TA that you have booked the cruise that you are on when booking or can you transfer booking to any cruise TA.

Yes, you can still buy Future Cruise Deposits with the benefit of extra onboard credit.

I can't help with the second part of your question, I have never booked on board.

Yes, you can transfer to the TA who booked the cruise you're on (and that TA might reprice the booking favorably to you). If my experience with Holland America is any indication you can't automatically transfer it to a travel agency other than the one who booked your current cruise. That can be accomplished later, but I'm not sure which party to call to accomplish that.