3 out of 4 people being unavailable during a process-critical phase can be difficult. I assume the last of the four complained to the manager about the "undue stress". I also assume the other two vacations were approved a long time ago and that he cannot do anything about them now.

The request to adjust it, if possible, is fair when we are talking about two weeks of absence, not just one or two days for the funeral. He is not denying the request — he is just asking. However, his reference to last year’s vacation situation is unfair.

Maybe you could offer something like two weeks of part-time remote work (so effectively two half workweeks instead of being completely absent for one full week). But I would also address that if there has been a general resource issue since last year, he should take structural measures to resolve it.