Well Mankins was the best Guard in the league according to one man, Dr. Z. It doesn't necessarily make it so, although I do trust Dr. Z's opinion on linemen over any other writer. But that's irrelevant, because, as you say, Grubbs would almost certainly start out at RG (although I wouldn't rule out RT).
I agree with 100% of what you say above, but that's not the issue. In an ideal world, we would get an ILB ready to immediately contribute and a starting-level cornerback in the first round. No question.
My starting point in this discussion though, is that if there are no guys at those positions there to be drafted at #24 who fit the Patriots needs, then they should take the guy who can upgrade their offensive line.
I would rather draft Grubbs at 24 if it comes down to him, a CB who doesn't fit, like Houston or Ross, or a CB or ILB who does fit our system but isn't a first round value, like David Irons or Stewart Bradley.
As it is, we'll probably reach at #28 for a guy who is a second round value, like Josh Wilson or David Harris.