I hope my predictions don't become self fulfilling prophecies
Well it's because you don't look at it in isolation. If grip work came 3 to 4 days before and then you didn't do anything else that is one thing. But the grip work and then the pulling, that is another. Not that that would always be a bad thing for everyone but obviously you had problems and it's best to assume it's not just a coincidence.
Another problem is just because forearm work alone won't turn you into a grip master that doesn't mean it won't affect your grip come deadlift day. It will.
There's nothing wrong with switching to the over-under grip when you need it. It would have meant you completing everything without all the hitches. It can be a little funny feeling...just keep the shoulders locked in. But one of the bigesst reasons I preach a pronated grip is not only because it trains the grip more...it gives you a "rescue grip" to go to, i.e. the over-under! If you always did the over-under your only way out would be to use a hook grip, resulting in excruciation pain, or put on straps.
Go ahead and practice with the over-under on other lighter things so you are more used to it, if that makes you feel better. It does pull on the muscles slightly different so it wouldn't be a bad idea to be prepared...but it's not a huge deal as far as I have seen. But having your grip fail near the top can be a hug deal because it can acually result in you body shutting down key muscles and you end up with a strain. Basically you could look at it like the brain getting a signal that the grip is failing, and since you are tying like hell to hold on, the brain has the very irrational response of saying, "Ok, hit the kill switch".
I switch out anytime I need to and I have to resort to the over-under plenty of times when doing heavy rack pulls other things that are just too heavy or too long or both...
P.S. Let me be sure we are talking about the same thing. I mean to switch to the over-under once you feel your grip is failing just to get your session done. Not PERMANTLY!