Before you contemplate surrounds, liners, or regrouting, you need to fix the damage that has been done behind the wall, under the tub.
Water has been able to get behind the wall and that is never a good thing to cover up. But, you might be lucky in one part of this because you said the water stains are on the ceiling in the garage. Is the tub over the garage or has the water traveled to the garage?
I would cut out a small area of the garage ceiling to see what damage you can locate from there. If it is over the garage there should be insulation between the ceiling and the subfloor. It's Probably wet and needs to be replaced.
I would also remove any of the tiles that are loose and remove any damaged drywall.
I would make sure all of the wood was dry and depending on the amount of rot, I would remove the rot and use wood putty to fill the gaps if there was not much damage or I would replace the wood if there were alot of damage.
Now, you can get to the rebuilding part.
I think I would try to go with the liner with the surround If I were not going to rehab the whole bathroom.(Being married, I would end up with the new bathroom.)
You should be able to get everything you need at the Orange store.