Yeah, Alex told me what I was overlooking: the ladder that leads underwater. I'd figured, "Hey, last time I jumped into water, I died, so there must be some underwater powerup I need to get later in order to climb down this ladder." Turns out that, no, you just need to climb down like any other ladder and you can mysteriously breathe underwater here but not in any other area's water, all of which just hides dead drops. Okay then.
So I got cat power and then I did comprehensive backtracking with the scanner accessory equipped to find all the items and then game finished. Short, but hey, indie production, no big surprise there. There are worse things you can say about a game than "I wanted more of it because it was good." And it is good! Fun gameplay, excellent pixel art environments and characters, and moody music. Game is cool and dudes should play it. If Symphony of the Night, Touhou, and Dark Souls had a menage a troi, I could believe this being the resulting child of questionable parentage. Lots of cute little touches--church lady's cats will meow at you while you're in cat mode, there's a self-poisoning accessory for Fenrir crazy people who like to play in Red Tearstone Ring limit mode all the time, and did I kill that one prisoner NPC just by talking to him? Also I actually felt a little bad for beating up the imps at the beginning.
Steam says I got the "True Ending." Okay. I'm not at all sure what the qualifications for this might have been! (I said Yes to both of the NPCs that asked me questions, if that means anything.) I know there are some things that I definitely missed in spite of an attempt to thoroughly explore everything. Notably: 1) there's a brick wall in the underwater crypt area that I'm sure is destructible, and the radar confirms there's a secret there, but I don't know how to get through; 2) the locked door in the monastery just at the start of the long ladder shaft; 3) the other locked monastery door in the upper right corner that just says, "??????," when you approach it. Guessing this is a bug unlock or something--I only found 18 out of 20.
I also spotted a Not!Jim in the credits list of Patreon backers. ^.^