Wow, that's a tough one. Is there someone in the cast who can bash out warmups on the piano? They don't really even have to be able to play, just to know some warmups and be able to hit the next starting note in the sequence. If there is, get them to come in a bit early and take warmups - I usually find I have at least one person in a cast who can take warmups (and the MD loves it - if he's running late I get a text saying "can you please get so-and-so to take warmup"!). And I think sievep is right - you have to stand up for your singers. It's difficult to do, and you have to be really diplomatic about it; can you get one or two of the more experienced cast members to back you up? I'd phrase it in as non-accusatory a fashion as I can - "some of the cast are finding their voices are feeling very strained, they would really appreciate it if you could give them some help with avoiding this." As for learning parts incorrectly, maybe use the fact that he's using MIDI and ask for the cast to be given copies of the files so they can do some work on it themselves with a guide. If none of this works, then yeah, you need to go to a producer or someone higher up.