I would confer with your tile installer. And why is he not doing this work? Most tile pro's I am familiar with will insist in doing the waterproofing.

remember when wet shimming or shimming you need to maintain full contact to account for 16 oc specs.

Plumb level and square is difficult,, it always requires floating the adjoining walls, it would have been spelled out in my estimate as an line item $$$.

Your walls aren’t square and plumb. You’ve got to make the Kerdi square and plumb. No matter what kind on nonsense is behind it.

I would trim the studs back appropriately so the Kerdi is even with the wall and the kerdi is square and plumb.

Looks like you are going to have to shim out the existing drywall/replace it with new shimmed drywall sheets to match your perfectly-plumbed Kerdiboard. Paint is forgiving, but the transition will still be a visual eyesore if you don't match them up.

Having done this twice now and looking forward to our third DIY bathroom project, it's honestly amazing how bad the finished tile can look if your walls/corners are even the tiniest bit out of plumb. Tile is a harsh mistress. Get those walls corrected now while you can, because even the most talented tiler will be hard pressed to make up for it using thinset alone. Your tile guy will thank you!

Your shower install should be turn-key by your professional tile setter. They should be a certified Schluter installer. Installing the Kerdi yourself likely violates their warranty.