Switched over to the release version of this level. It makes it possible to climb out of the final area. With that, 100% completion became possible for me again, so I decided to try reaching the top secret place only meant for DonDoli and Gliperal as a reward for hunting for void screens. Unlike those two, I'm a human being, though. So it took me a while to get there. At least it's a rather amusing secret.
Also watched the credits again to see if they were updated. They were indeed. Though the last image has some of its text covered by a button.