1.) I would drop to 22.25. 2.) You could drop it, but keep in mind that lower back "fullness" (as you call it) often won't be alleviated by taking in the side seam. You could also add darts, which should significantly help. Otherwise the shirt looks good. Stop fretting.
I'm 13 PC shirts deep and won't be stopping anytime soon.
In terms of the new layout - I like the search bar - I always remembered fabrics via what they were called and generally always forgot to favorite them. I'm sure you've heard this a million and one times already, but I wish that the shirt "simulator" on the left hand side had more options than the traditional button-down, English spread, point, and Londoner. Knowing it might be super crazy to have all of them,...
Purchased the cardigan and pants a few days ago, getting them in today. This is my first piece of knitwear from S/S, and am surprised by how slim the fit is (I bought a M at 6' and 190). Additionally, I'm equally frustrated at the inconsistency in the pants sizing. This is my third odd pair of pants, and all have fit very differently (one perfectly, one decently, and the linked one poorly).