Rule Trivia
 

R1 on first, no outs. Batter hits to left field. R1 rounds second large, but decides not to go. BR doesn't see R1 return and is also heading for 2nd.

R1 is obstructed by the SS returning to the base. Throw comes into 2nd base ahead of R1 (or R2 for that matter), R1 takes 3 steps toward 3rd, ball is thrown to F5. As the ball is thrown, BR continues to 2nd base. R1 returns toward 2nd, BR stays there. F5 throws to F4 as R1 slides in safely. F4 then tags BR.

Ruling?

BR is out, R1 was entitled to the base. R1 is at risk to be put out between the bases obstructed (if she leaves second) as there has been a play on another runner and she reached the base she would have reached if not for the obstruction.

So ... just one opinion here so far? Really?

Well
 

Once an obstructed runner is played on by the defense the ball becomes dead. At that point you then award bases. The way I read the OP R1 was played on sliding back in to 2B before a play was made on BR. At that point, the ball becomse dead.

I get the sense that you are posing a conundrum about who is entitled to the base. Should be first to acquire and not forced. So,
a.) did R1 "acquire" 2B by rounding and returning?
b.) Or does BR "acquire" by being already on the base when R1 returns?

I go with "a" and the previous posts, BR is OUT.

I thought it was retired.

Ball isnt dead unless the obstructed runner is put out prior to reaching the base they would have reached absent the obstruction. A play was made on the obstructed runner, but she was safe diving back into 2nd. BR was then tagged while also on 2nd. I also have the BR out, based on the description in the OP the runner was obstructed returning to 2nd and she was safe, no dead ball, no base awards.

It is.

What about the fact that when the play is over we are to announce the obstruction and award the runnerS (plural) the bases they would have achieved had there been no obstruction.

Had there been no obstruction on the runner, she would have made it back to 2nd before the BR got anywhere near there. We cannot and should not assume BR would have been put out - it's much more likely she simply returns to first.

Ahhhh...there is the real question.

When I read the OP, it seemed pretty straightforward that the BR would be out, so I was looking for what the "real" question was.

Since the obstruction rule says that we can award bases to the obstructed runner and any other runners affected by the obstruction, do we consider the B/R as "affected" and place her back on first?

Re-create the play, absent the obstruction. As I read it, BR would still be on 2nd without paying attention to R1, the only difference is R1 may or may not have been safe back into 2nd on the initial throw behind her; or maybe gets out of the rundown quicker.

In what way was the BR affected? Poor baserunning put her in jeopardy, not the obstruction on R1. If R1 is legally put out (absent obstruction), BR is safe. If R1 gets back safely, with or without obstruction, BR is out when tagged.

I agree...I was just trying to state the unasked question that I think was the point of the OP.

After thinking about this play a little longer, I came to the conclusion that R1 started this whole thing by "rounding large" and getting herself in a rundown. She was (potentially) bailed out of the rundown by the obstruction, but if she had not made such a large round of second, it's probable there is no rundown and B/R stays at first.

I suppose, then, that what this one really comes down to is - in the umpire's judgement was BR so oblivious that she would have continued to run toward 2nd - or was the majority of the run to 2nd due to the run-down in front of her. I guess the way I worded it and absent more information regarding BR and umpire's perception of her actions, this one could go either way.

Honestly, I don't see how either oblivious, or intentional, it was for BR to continue to 2B, would be a factor in the ruling. (oh and don't forget the 1B coach in the oblivious category).

What I may have missed in the OP was the results of the OBS.
Would R1 have reached 3B without the OBS?

While I could see an entertaining discussion arising out of that, but even if yes, I would still have R1 out. The smarter play would have been for R1 to get tagged out off the base, then we have Dead Ball, award R1 to 3B and nobody would have tagged BR.

Why because BR was tagged out which ended the play. R1 would then be awarded 3B, the the out on BR would still stand.

I don't think that's what's meant. What we're asked to do is not piece the play together completely as if there had been no obstruction but to determine who did something different because of the obstruction.
In this play, R1 is going to be back at second safely absent the obstruction as determined at the time of the obstruction. Therefore, BR is not going to get second absent the obstruction. If R1, realizing she has been obstructed decides to try for 3rd, the BR may decide to advance to second. But she is certainly not protected in doing so despite the fact that absent the obstruction she does not attempt to take the base.
Now, it's likely instructive to consider a case where the runner being obstructed clearly does make a difference to BR.
R1 is obstructed coming around second on an obvious double to left field. Realizing she can't make third now and unaware the umpire saw the obstruction she returns to second causing the BR to return to first. After the play award both runners an extra base.
Is your case more like my second or my like the first? I'd say more like the first. Yes, had R1 not been obstructed it is possible that BR would not have made the same baserunning mistake, but it's too much of a stretch for me.