Bathtub replacement

I have been picking up hints and tips from this forum for a number of years and wanted to say thanks to everyone. I decided to undertake a fiberglass tub replacement with a Sterling Vikrell tub/surround. I figured the demolition would be the tough part, only to find that the builder had cut the subfloor under the tub a little too far back so that it wouldn't support the left end of the tub. I only need to extend it a couple of inches and was looking for ideas. Check out the plumbing job done by the builder in the attached pic, notice how the tub drain is attached to the trap and how they toasted the PVC. I assume I can cut the old mess off back to the straight pipe and install a new trap adding a compression fitting for the new tub drain?

Nothing really wrong with how he connected the trap, it just takes more work to get it right in most cases.

I would cut out a little more of the subfloor so you have straight lines to work with. Then add blocking to the existing joists so that a new piece of subfloor can be installed with good support under it.

I am thinking of adding a layer of 5/8 plywood leaving a smaller opening for access to the drain. I will also add some reinforcement under the area that was cutout too much. There isn't much room to add blocking. The adjacent tile is 1 1/8 inch higher than the subfloor, so I think this is okay but was curious what others thought.