I think it comes down to a few things, the biggest one being whether Carr has already signed a contract to be the next head coach with a rolling start date. And that MAY have already happened a while ago - quite possibly during season 2023.
If that is the case, if Port want to give Hinkley a further extension (for 2026 and 2027), then the issue becomes one of how can anyone in their right mind have signed a senior coach to a contract that wouldn't be enacted for 4 years AFTER the contract had been originally signed. The alternative would be to rip that contract up, in which case the payout could be close to 2M.
The Board are in the stickiest of situations if they want to play it out like this, and it will be a situation entirely of their making with no one else to blame.
If that rumoured contract with Carr is, in fact, there, it sounds like there is a real risk to be unenforceable for uncertainty.
I'm not saying there aren't ways to draft a contract of that type which has the potential to be binding. It's just that it would take a fair amount of work.
Of course, in the hypothetical you paint, it is Carr who would be seeking to enforce the contract.