The premise alone put a huge grin on my face, and the rest of the adventure doubles down; this is a beautiful way of handling devilry in an OSR game! Morpheus would be really fun to roleplay with his inability to lie. He bleeds that over-controlling DM energy, the whole "how dare you PCs mess up my story!" vibe. And the fact that you can literally deal damage to him by being creative is spot-on. A very imaginative adventure!