HTBT. As as soon as there is OBS the umpire must make a judgement, clean BH to LF, fielded cleanly R2 not going on contact for ex. might be protected to 3B? Soft liner over SS, with a shallow LF, R2 heading back to 2B, maybe protected to 2B. Scorcher down the line F7 diving stop with two out R2 going on contact, maybe protected to HP?
You just can't say, R2, single, he's scoring, you gotta see the situation and make some judgement/s.
And yes, if the ump protects him to 3B and he rounds it trying to draw a throw or something, waving to the GF in the stands, slips and falls on his own, then he is at risk to be thrown out upon his return to 3B.
Where as if the play is close, with throw driving R2 and F5 to the outfield side of 3B. In an effort to avoid the collision,, R2 attempts a wide hook slide, touches and goes beyond the bag, I'm probably protecting back even though he got the one I was protecting him too.
Otherwise, pass the one the ump judges you get, your at risk.