Hi!
Yeah, chapter 5 and 6 are shorter (I might try to expand chapter 5 a bit with some short quests, but there aren't going to be many big changes).
I am glad you liked the story. The scenes are kind of their own thing, as you noticed. There are various reasons for that, but probably the biggest one is simply that this was my first open(-ish) world game, and I tried to give it variety and make as much of it accessible from the get go, so it ended up being a patchwork of various ideas.
The alterations do not show up in the CGs, only in the "default" busts that I use here and there (it wasn't part of the original budget, and afterward it would have been too expensive and awkward to try and "patch it in").
Getting all the statues doesn't lock you into a bad end (although it makes one available). You get a different branch gameplay-wise in the demon world, so if you are interested in a slightly different castle section or if you want to see Erica jill-off to her obedience to the demons, you can go for it. It's not a major ending, however (as you noted, it's more similar to a nonstandard game-over).
Smoke screens and Yellowpetals are unique to the new store, if I remember correctly. Did you want something different available? If it's not a huge overhaul, you can give me your suggestions, and maybe I'll take a second look at the stores, the items available and/or the interactions you have with each one.