Finish a wall with uneven stud surfaces
If it were my bathroom I would finish the wall with framing the way you have started. I perhaps would rip the 2X so it is tapered so I would not use so much wood. After that is done and drywall is up I would remove the 1/4 round at the window jamb and replace it with a jamb extension that would meet the face of the new wall. I would also "tweak" the dimensions of the jamb extensions so the taper is not as drastic and let the differences form the wall face get hidden by the trim. 

In my opinion, it is much less noticeable to put the difference in the tweaked window jamb than have it where it is now.