Would a grandmaster and a computer play better than a computer?
Not a dumb topic at all. I suspect yes. Much of correspondence chess has given up trying to keep folks from using engines; the best players don't blindly play whatever the engines propose, but use their own experience as well. As I understand it, this often involves prodding the engine to consider alternate lines. Hopefully someone who knows more about it will comment.
In 1997 (when Deep Blue beat Kasparov, allegedly with some human interference), the answer would probably be yes. Computers were strong then (not nearly as much as now of course) but they still made questionable decisions, and having a grandmaster to guide them in choosing moves could make them play at a significantly higher rating.
Nowadays the engines have enough raw computing power + better hardware + sophisticated algorithms take all the external factors into account and so computers can easily beat humans in every aspect of the game!
I can't find it, but I've read somewhere that this is indeed the case -- that GM + engine can rack up a pretty good plus score against an engine alone. But since, as I say, I can't find where I read this, still less, any citation of evidence, I'd very much like to hear of any evidence there is of this one way or the other. It sounds intuitively right to me, but I'll defer to any hard evidence anyone can provide. (Surely the question has been tested, yes?)