Protection Between Bases with No other Runners

[Manny A]

Haven't been here in a while, so if this has been asked recently, just point me to the right thread.

My question has to do with the Obstruction rule in most rule sets that says a runner who is obstructed cannot be put out between the two bases where the obstruction occurred, unless one of a number of exceptions takes place. One of those exceptions stated in the FED rule book is as follows:

1. When an obstructed runner, after the obstruction, safely obtains or returns to the base she would have been awarded, in the umpire's judgment, had there been no obstruction and there is a subsequent play on a different runner, the obstructed runner is no longer protected between the bases where she was obstructed and may be put out.

What if there isn't another base runner? Does this exception no longer apply? Here's a play to discuss it:

BR hits a base hit in the outfield, and is obstructed by F3 as she rounds first base. She returns to first safely, and the BU decides that the BR would not have reached second base without the obstruction. The ball is thrown in to F6, and she casually walks it back toward F3. Nobody is covering second, so the coach, thinking her runner is protected between first and second due to the obstruction, tells the runner to take off for second. The defense notices, and F4 runs over to cover the bag. F6's throw to F4 retires the runner at second on a close play.

Does the runner still have her protection between first and second here? She did make it back to first base safely, which is the base she would have been attained had there been no obstruction. But there was no subsequent play made on another runner, taking away that protection between the two bases, because there were no other runners to make a play on. In this play, I would send the runner back to first base.

I can't find a case play or clarification anywhere that says the between-base protection goes away if she makes it to her trail base safely, and then gets caught trying to advance to her next base when there are no other runners that may be played upon. Is there something out there that says there must be at least one other runner that may be played upon for the obstructed runner to maintain her protection between two bases?