RBI or no

Yes. It is scored as a fielders choice with an error. Since they fielder chose to make a play at 2nd, the run counts. (You can't assume a double play). Had there been two outs then no RBI

Would you score that a sacrifice?

True, the runner reached 2nd only due to an error, but the runner from 3d scored on the play.

Seems a shame to score an 0-fer AB on that play.

By rule, the runner needs to have been put out (or would have with errorless play) at first in order for this to be scored a sacrifice. 10.08a

If the play were made to first and the batter-runner safe on an error, the batter would get the sacrifice. The situation above is a bit different but given that the run scored I was wondering if he could get the sac here.

Should be reached on FC for the batter in that situation.

True, but not an answer to the question. The question is, whether or not he reaches on an FC, does he get charged with an AB, or does he get credit for a SAC?

If the defense chooses to make their attempt on the runner going to 2B, there are only 2 options left to score it, fielder's choice or 1B. Their attempt must be on the batter to make it a SAC. If in your judgment, the reason they went to 2B was they had no chance to retire the batter, then it is a 1B. Otherwise, it is FC.