Well, we've already established there's no method to their madness - only madness. You make a good point about the chance that Griphook knew what they were up to the whole time. I find it quite likely that whether he knew or not, he didn't trust them as far as he could throw them, and planned accordingly.
My mental image of the cup, never mind the book's claims to the contrary, never includes handles. It tends to come across as one of those... I think they're called pilsner glasses? The tall ones for beer that're wider near the top? (Don't mind me, it's Late.) In any case, Harry's grand plan would've been that much grander if he would've taken the action then, rather than just make off with the thing and let Ron and Hermione take the action later.
(I need to re-watch it myself... if only to fuel the damn Highly Improbable Crossover this thing started. gehayi and I should not be allowed to plot. XD)