The blunders computers make and that human players make are/feel very different. I wouldn't expect to last long if I were cheating on, say, lichess.org by playing out the moves of a mix of engines at various ratings.
Basically computers can give good hints for you. Pretty sure this type of thing would be undetectable.
It probably isn't as helpful to a grandmaster, but that's true of engine analysis in general
Yeah, although at that level lots of people are also getting into good positions, having no idea why it is good, and losing it.