Is this a real example? Are there actual rumors of this?
In any case: Of the names you mentioned, I think Thomas Kail would have the LEAST amount of say over it. I don't even think he would enter into the equation.
After that, it sort of depends on how you interpret the question. The casino owners have every right to say "we're going to let you perform here if you don't shorten the show." But the Hamilton team can always just say "screw you, we're not going to perform here in that case." They have that power. The casino, in this case, is like a landlord, and the landlord can't force someone to rent from them if they don't want to/have to. And in the case you're describing, I definitely think the Hamilton team would walk away. The casino needs Hamilton more than Hamilton needs the casino.
But let's say the Jeffrey Seller wanted to make the cuts. I'm actually not sure if he gets final say, or if LMM gets final say, given that LMM presumably still has creative control over the material? I actually don't know how that works. If we were talking about a licensed production (like though MTI, R&H, etc.) then I think the author would have final say. But I'm not sure if the rules are different with an original production getting re-mounted by the lead producer.
But I will say this too: even if Jeffrey Seller technically has final say (and again, I don't know if he does), he still might not have the bulk of the power. LMM is a more visible public figure, with star power and public admiration. If he were dead-set against the cuts, I don't think Jeffrey Seller would dare to cross him in that way, even if it were TECHNICALLY Seller's call to make. In show-biz, the power balance doesn't always favor the highest on the corporate chain.