Drywall and tile to top of shower

I don't know exactly where the other wall is you want to match up with. The standard method is to shim out your rough opening to be exactly what you need for the tub. That means putting shims on the studs around the tub to get it lined up where you need it.

Once the tub is in where you need it, add more shims to the studs above the tub to bring your backer board even with the tub. If the lip of the tub is 1/4" you will need to add at least 1/4" shims the entire length of each stud (maybe more if you have more space on some studs). I used wood lathe for my shims, if you have a table saw you can easily make these shims yourself to whatever dimension you need for your tub.

You just want a stable platform for your tile. It depends on the size of the tile.. If you are cantilevering 10-15% of a larger tile, just fill the thinner areas with more mud and put backer where it will fit.

It looks like you needed some blocking behind the green board (where the rear curve comes in) to be able to fill in. Perhaps some fiberglass mesh tape would let you land some thinset to bridge the gap.