Yeah this isn't Enter the Dragon where the fighters are trapped on some island forced to fight.
If by some miracle they ever did a deal its going to be longer than 3 fights. They will lock him in. Jones signed a 8 fight deal right before the Gane fight. UFC will not make the same mistake with Ngannou twice.
Main issue as you mentioned earlier is Ngannou probably will not be active as he is wealthy now and older. He will be closer like a Jones 2.0 which given the shit state of the division is better than nothing.
I still have major doubts they can get the deal done. If Ngannou just did the Tyson fight and the PFL Renan fight I can see major pressure to get him in and Ngannou having bargaining power. The Joshua fight really fucks things up. This is the outcome what the UFC did not want all along with the boxing thing....their HW champ getting brutally slept in 2 rounds. They did it with the McGregor/Mayweather as that was #2 or close to the biggest fight in history and a retired smaller Mayweather wasn't going to KO Conor like that. But they did not want anything to do with their HW champ fighting bigger active HW champs for far less money. Now that Ngannou has a lot of money I don't see him wanting to come negioate down to get into the UFC at this stage. I would like to be wrong on this but I don't think they get this done.