Looking for some advice on an issue with a "wall thickness" issue I'm dealing with in a shower rebuild.

After teardown to studs of the existing shower, I can see that in previous work done on the house the original plaster/metal mesh make up of the walls (house built in 1959) has been replaced by a double thickness of drywall halfway down the walls. As a result the walls are not plumb, as the double drywall portion of the wall (bottom half of the wall) is approximately at least 1/4" thicker than the top half, meaning that if you're standing at the entrance to the shower the walls slant like:

\ / (luckily not at that extreme of a slant, but you get the idea)

The problem I now have is how to compensate for this as I rebuild the new show walls, as they are level and plumb in order to accommodate the new large format tiles and shower doors I will be installing - having a small subway tile and curtain allowed the previous build to cover up these inconsistencies.

So my question is: how do I bridge the gap where the new wall thickness of the shower does not meet up consistently with the thickness of the walls that meet it? drywall compound and a lot of feather and sanding to try to build up the low parts of the wall to meet up with the shower? Help!