Andrea, I might have missed it, but I don't remember if you said that any of the tiles in the shower that were loose. Are there any soft spots in the shower? Check for soft spots by hitting the tiles with the heel of your hand. If you hit up at the top of the shower, where the tiles are most likely alright, then you'll know what you're feeling for when you get lower. If they are solid, I don't believe I'd remove them. I'd only remove them if the backerboard is soft. If the backerboard is solid and the tiles aren't loose, I'd remove the grout in the areas that are cracked and regrout those only. Maybe that will fix you up until you are able to afford to re-do the shower later. If the tile is very old, you might have trouble finding replacement tile to match. If you remove the tiles and don't break them, you can clean them and put them back on. It's tough to scrape the mastic off the back but it can be done with a chisel.
As everyone else said, the drywall in the other room needs to be removed. That will tell you a lot once you get in there.