HP, I have read enough about your sich to understand your W is a drama queen. Still, I'm going to put in a plug for her on one point. If I had been trying to get in touch with my D12 and she wasn't answering, I'd then go to the adult in charge. If the adult declined my call I'd be ticked. I wouldn't have expressed it quite the same way, though. So.....maybe just ask your S to call mom after school? You don't need to say anything to your W about it though, imo.
But on the schedule change, in my sich I'd say something because H and I have specifically agreed that all schedule changes have to be made between the parents. There is to be no using our child as a messenger by either of us. If H made arrangements with D12 without going through me, I'd make sure she wasn't available. But if H asked me about it first, I'd accomodate if I could. I'm doing it this weekend for him. I think you and W just need to clear up how changes are handled.
