As someone who has recently figured out how to get the corrupt endings, I can give a bit of advice. I believe the key points are as follows:
1. You have to do the scene of rampaging in the first human village, the one with the mayor. To unlock this, you need to absorb all of the villagers, be at size 450+, AND have met Lucile in her lab. This isn't to be confused with completely destroying the town like you can when you're at size 600+, you need to be in the town map and have the encounter with the real mayor.
2. You need to have done all of the corrupt actions (including destroying both main cities and the fairies) BEFORE you fight Lilith in the ruins of Fimbulvetr. You may also need to be at the max size (2000+) for this to trigger. You'll know it worked if Isabelle fully absorbs and crushes Lilith.
Things did get a bit buggy at points for me, but I still got through to the endings just fine.
After getting Lucile powered up, not going corrupt, and losing to Luna, she appeared in the party with Isabelle at 1hp. This was on the overworld and during the final fight. The sprites in that final battle also got pretty weird. Isabelle suddenly flipped upside down when Luna started growing.
