If Khamzat stays at WW, I got no problems with him being next after Usman, except that he should make weight again before he's booked for it. Best solution there that doesn't involve risking him in another eliminator is have him make champ weight as a backup on the Edwards-Usman card. If anything happens to the main, he's already proven he's ready. Even if he doesn't fill in, he's demonstrated he can make weight. If he doesn't make weight, then the decision is settled and it's off to MW he goes. And if he can't handle himself backstage with Leon and Usman there and causes another problem, then he's demonstrated that he can't handle himself at all and the UFC can deal with him.
If he goes to MW, he should get the respect that beating Leech, Holland, and Burns should get him at MW. Doesn't mean he starts over, and doesn't mean he's #1 either. And the reason he shouldn't fight Vettori, Costa, or Canonier is the same reason he shouldn't fight Colby. Regardless of how good those fights are for him, the potential benefit for both opponents is not equal.
When you book a fight, one guy wins and one guy loses (mostly). And that's equal, on a base level. However, momentum is a question in the rankings, and whereas Khamzat winning any of those could put him in instant title consideration, beating Khamzat shouldn't do the same for them. You don't take repeated gambles like that, especially in a division barren of contenders, because if you lose, you lose bigger than what you would have won