You just typed an entire post telling me why he can't do it and then said the bolded.
I'm not saying he's ideally a 155lber. I even said there's a reason he fought most of his career at 170. I'm saying he could do it and I think he would for a chance to be the first ever to be a 3-division champ. GSP thinks he can do it, too, so....
If the fight is made, I'd put my money on GSP. GSP is not the fight anyone, anywhere, any time, any weight class type of risk taker. If he accepts the fight, it means he thinks he will win. In this case, that would include cutting weight without diminishing himself too much. And IMO, GSP is too well rounded for Khabib (or anyone, really). Even if he's not quite as good of a wrestler as Khabib, he's going to have better TDD and ground game than anyone Khabib has ever faced in the UFC and he's a better striker than Khabib.