The save in the final boss room was actually a mistake haha. I put it in for development, and did actually remove it from the level files, but the Godot importer didn't pick up the change. As you say, I completely ran out of time so that boss is pretty terrible. The accidental save might be a blessing in disguise!
The invisible damage is due to the missing particles issue. You should be seeing steam jets but unfortunately they don't render on the web build. Thanks for pointing it out, and I appreciate you making a video. I can't fix it at the moment because of the jam voting period, but I'll try to patch it after.
It's a bummer that the download doesn't seem to be working for a lot of people right now. Seems to be an itch.io issue.
Thanks for the feedback!