Your suspicion is correct. You choose what land to return to your hand on the resolution of Rakdos Carnarium's triggered ability. You can only activate Bloodstained Mire when you have priority, and no player has priority while an effect on the stack is resolving.
December 30, 2014 6:28 p.m.
Thanks I figured it out after thinking for a couple of minutes, (I'm pretty slow), but our comment was a great clairifcatin :)