Not going to lie, I'm not a fan of fighters or coaches using Ramadan as an excuse not to fight. At all. So you have to train differently for a month. That's your choice and your problem, shouldn't be anyone else's. MMA's not a 9-5 Monday to Friday gig anyways, so rub some dirt on it and get to work.
Chimaev took no damage at all in his last fight, no reason he can't fight. January would have been a good date for in interim fight if KC was a plan for the WH house card. Otherwise, the WH date aligns almost right on schedule for the one-fight-a-year-champ.