f somebody is doing Y when Sensei demonstrated X, why isn't Sensei correcting them?
At my last lesson while attempting to learn the Omote version of a technique I kept mistakenly moving so that I would wind up doing the Ura version... rather than correct me, Sensei just showed us how to follow through with the trow and pin doing it that way. Oddly enough once I saw that version I was performing the Omote wit less confusion... sometimes its good to just not fight what your body wants to do and go with it and learn from it.
My partner that night is what you call a rebeginner. I really like how he is knowledgeable enough to help me learn and yet humble enough to realize he is not where he once was and willing to accept the adjustments even from me on the rare occasion when I am able to recognize something not quite right.
As for talking on the mat. I'm afraid I'm guilty, but not so much of giving advice.. because I really can't yet. But I talk myself through the movements or mutter about the mistake I've just made. Its part of how I focus myself on specific things when I'm nervous. I am grateful that my fellow Aikidokka simply smile and continue on and are not (visually anyway) irritated by my chatter. Usually they don't speak in return but continue to show me what to do and that's actually ok. So if someone offers advice to you let them.. it may be they are helping themselves more than they are helping you. There is no law that says you have to respond in kind.
Sometimes I have to tell Sensei, or my partner, that I am having trouble doing things they way they are asking (generally related to my knee issues). But I do at least try it before I decide its not working and look for another way.