I would not try to repair the tub. Even if someone felt they could do it, it would likely have removed. By the time it was repaired and reinstalled you'd have the cost of a new one invested and no real assurance that the patch would hold. The shower stall leak has ruined the floor and will have to be totally torn out. I suspect there is considerable damage under there, but a little damage or a lot of damage, you will have to get everything ripped out to fix it anyway. Yes, it is likely the drain was the cause, but you might be able to tell more when you get into it. I fear there are a couple of pretty hefty remodeling bill headed your way.