opening subfloor in living room ceiling to access 1st floor bathroom pipes

[Kosh Vokter]

I have a water leak in the living room ceiling area, just below the tub from the bathroom upstairs. A plumber cut the drywall and detected the water leak coming from copper pipes that's connected to the bathtub in the bathroom upstairs. The copper pipes need to be replaced, but accessing the copper pipes has been tricky. The plumber said he has to cut more of the subfloor to be able to access the copper pipes, but he was reluctant to do it because he said something about that might affect building code or might create instability and suggested to get a building contractor. The bathtub is not usable until this is fixed.

I got another plumber to come in and he said he can cut some more of the subfloor and access the pipes, he wasn't too concerned about a bigger hole through the subfloor. the subfloor is already cut for about 10x10 inches, but that's too small an area for the plumber to reach the pipes. 

Question: How big of an issue is this, if we have to make the hole in the subfloor bigger? I don't know if I need to be cautious like the 1st plumber said or go with the 2nd plumber's recommendation. The alternate options mentioned below are likely to be much more expensive. 

Alternate options considered:

1. cut the fiberglass bathtub from the bathroom upstairs to access copper pipes - need to get someone for cutting a hole through fiber glass and fixing it back up after the copper pipes are replaced.

2. cut the drywall in the bedroom upstairs adjacent to the bathroom - the bathtub and copper pipes are deep inside might still make it harder for plumber to access

3. pull out the master tub, remodel and rebuild the bathroom upstairs.