Award after obstruction on another runner

[blueit]

I check this forum occasionally to stay up to date on rules issues, but haven't posted. I'm a 5th year ump, doing mostly high school & some ASA. I have a question about what to do at the end of a play on which obstruction occurs & is called.

R1 on 3B, R2 on 1B, 1 out. Fly ball to LF. R1 tags & tries to score, F2 sets up directly in basepath without the ball, runner veers toward foul territory & attempts headfirst slide, ball arrives just as R1 bounces off F2's legs & misses plate due to the contact. F2 hits the dirt, too, but scrambles to tag runner behind the plate & does so before runner can touch the plate. Plate umpire has signaled obstruction & calls "Obstruction -- safe!" as F2 attempts the play on the runner.

Meanwhile, R2 has tagged, reached 2B & rounded it. Seeing the contact at the plate & F2 going after R1 behind the plate, R2 heads for third & is most of the way there at the time F2 tags R1. No play is made on her at 3B.

Here's the question: does R2 stay at 3B or must she be returned to 2B?

I'm handicapped in not having my rule book with me at the moment, but I believe the rulebook reads that at the end of the play on which obs occurs,
the delayed dead ball becomes a dead ball & runners are awarded the bases they would have reached, in the umpire's judgment, without the obs.

Here's the problem: without obs (i.e., if F2 makes a clean tag on R1 in front of the plate, or even if she misses the tag), R2 would never have tried to advance to 3B. But due to the action & delay in between the time of the obs & the time "dead ball" occurs, R2 is able to advance most of the way to the next base.

Should the base award be made (a) as of the time of the obs, in which case
R2 is put back on 2B, or (b) as of the time the ball becomes dead, in which case R2 is put on 3B. Seems to me that (a) penalizes the offense for an infraction by the defense. But I supposed looked at the other way, it's a question of whether the offense can gain an extra base as a result of an obs infraction by the defense.

I hope I've explained that clearly