I believe this is a difficult play to officiate, and there have been good points made on both sides (interference, and not interference). The retired runner clearly hindered the play (I found out last night that he was hit by the throw, even though he was ducking to get out of the way), even though he did so unintentionally. Since this happened on a force play, I don't think we can call out R2. I'll use another example to clarify this point:
R1 & R2 with no outs. The ball is hit to F5 who steps on third base and instead of throwing to first, he throws to second to retire R1. R2 doesn't slide and stops 10 feet from the bag thinking that F5 is going to throw to first. F5's throw hits R2. In this case, we would call interference and call out R2 and the BR and send R1 back to first. Why would the original play be any different?
A well-respected D1 umpire here in CA said that the OP would be nothing unless R1's interference was intentional. I disagree with this interpretation even if one were to consider this interference by a retired runner. If you were to consider it that kind of interference, I would still require the retired runner to be aware of what was going on and make an active attempt to stay clear of the play. In other words, I would consider indifference to be synonymous with intent. Just because the defense didn't make the play that the retired runner thought they were going to make doesn't mean he can be oblivious to the play they actually made.
My personal thoughts align more with Maven on this. The safest thing to do on a force play is to slide legally. Doing anything else puts you in jeopardy of being called for interference. While the runner thought he was running in a direction away from the play, he didn't. He ended up running to a spot that put him exactly in the throwing lane for the play F6 decided to make. It is actually similar to a video the NCAA used to show FPSR where R1 ran inside and F6 actually stayed inside rather than running outside where he normally would have been. R1 guessed wrong and paid the price. I have FPSR on the OP. R1 and the BR out, and R2 is returned to second base.