How far down should sheetrock come in a shower tub installation

Depending on where the crack/hole is, I would wonder about the support underneath the acrylic of the tub. Was the crack/hole caused by some part of the tub that was not supported getting flexed too much under regular use? If so, then I might doubt the repair kit myself. However, if the crack/hole was just due to some sharp object hitting the tub, then I suppose it will work fine. However, if it is a support issue, you might as well try the repair kit, because your other alternative is pulling the whole thing out to try to fix the lack of support, which sounds like a huge job. Just my $0.02 worth. Hope this helps.

It sounds like one of your valves is not working correctly. It could be that the handle is slipping or that the valve is clogged. Does it feel like the handle is turning the valve, or is it slipping? If its slipping you can probably just tighten the screw.

As far as replacing it, you might get lucky and find a replacement valve, but in the event you don't, they make remodel plates which allow you to relatively easily replace a 3 handle setup with a 1 handle faucet. The remodel plate covers up the area in the wall/tile you'd need to remove to swap it out; you don't have to tear out the entire wall. If you can get in behind the shower you can probably do it without the remodel plate.