Must be a lot of video in general of him right, no?
In almost every video of him playing he uses a last second grip adjustment as you call it, in at least a few shots.
Granted I have seen a few posts where you say you met and saw him play in person and I am not trying to discredit your account or opinion.
I would like to share a personal experience, my grandfather was a life long pool player, started as a young child setting up pins in a bowling alley and was allowed to play on the pool tables when there wasn't customers at the lanes, he played into his late 80s
He was born in 1926, and saw Mosconi multiple times in person, he always told me that Mosconi had a slip stroke. My grandpa would also use a slip stroke, would set up with his hand forward at address just like Mosconi, then would slide or "regrip" his cue during the stroke, he often put talc in his hand that held the cue so his hand would slide better.
Certainly my grandpa could have been mistaken, but he was a knowledgeable player who could run 100+ balls at any given time when he taught me(his late 70's then), I don't see any reason he would lie about that in passing.