How to install tub into non-square rough opening?
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.