Firstly, I think everyone agrees with you that Umar should be next. But you're blaming the wrong person here. Yes, UFC champs don't pick their opponents, they fight who the UFC tells them to fight. Completely agreed. But THEY DO get to dictate (somewhat) when & where they fight.
For example, Ankalaev was reportedly offered the Pereira fight @ UFC 307 and turned it down (we will never know for sure), because he wanted Pereira to fight him @ UFC 308. IF that is true, Pereira is in no way shape or form required to fight when & where Ankalaev wants. Pereira gets to tell the UFC when & where he will fight, and the UFC then give him an opponent. That's how it works.
So if Merab says "I just had a tough fight, I want to take a few months off and start training camp in January, and fight in March/April" then it's up to Umar to suck it up princess and fight Merab in March/April when Merab wants to fight. If that runs into Ramadan, then tough luck. Either quit the UFC or join the civilized secular world and fight or do training camp during Ramadan.
And you are completely wrong about Merab needing to sit out. The the UFC is not obligated to force Merab to wait for Umar if they don't want to wait. If Merab wants to fight in March, and Umar says no, then the UFC will find another opponent for Merab. It won't be the most deserving opponent, but that's UMAR's fault.