The 0 times this has happened in 15 years I am ok with either of the calls. If I make it or my partner such is life. I won't lose any sleep and neither will anyone else in the game, in the association, or the state most likely (possibly even the world).

The best part about it is if you make one call supported by rule then you can say "blah blah it's supported by rule" and you are good to go. Or if you make the wrong call you can say "crap never seen this and I kicked it" and you are still good to go. Ideally I would like to err on the side of rule support but this doesn't happen. The rarity it may happen in a meaningful game you won't get crucified (well you will get crucified by half the gym depending on your call or lack of call).

But was the contact relevant to the advantage? Did the coach, in conferring with the player, physically prevent the player from falling OOB. Or was whatever advantage the player gained independent of the touch?

I believe the advantage being referred to is specifically about physically aiding the player, perhaps preventing them from stepping/falling OOB.