Good CRM to me is not an FO that "keeps quiet" and all of a sudden takes a decision that the captain has to comply with on the spot. As you said yourself "you decide not to comment". Why would you do that? Where's the "crew" in that situation? Why would you wait up to decision point and "attack" the captain with a split decision "out of the blue"?
Good CRM is not calling out for a required go-around, it's calling that his speed is too high way before that because if he is not correcting at that point. You did fly the approach together. You had to be monitoring him. He might simply be fixating on something else... a captain is only human and very capable of doing this.
(ps: I'm an F/O if you would doubt that, but in these situations you can perfectly immagine the F/O having the same problem. And I hope my captain will not shut up and wait up to 500ft AGL to throw a go-around call at me...)
Case 2 is very different. If you were close the ground, pitching down 15° and 3000fpm descent... ever tried this with a passenger jet? (I HOPE NOT).