Demon questions

I have some questions about demons I couldn't readily find answered anywhere.

If you're chaotic, is there any advantage to summoning a peaceful demon other than that meaning you won't have to encounter them later on if they are unique?

I accidently summoned a peaceful Yeenoghu. It did not appear tameable, though since it will follow your character around is there anyway to use that to your advantage? Would a ring of conflict make it attack other monsters, or just you?

If you kill Death 255 times (on the Astral Plane) does he stop coming back to life? The same question with the Wizard of Yendor, or really any monsters that return to life (trolls, zapping corpses with a wand of turn undead, etc.)

One time I ran into Jubilex (helpless) one level above where she normally shows up. I don't think this was a bones file thing, is there any good explanation for this? (It was my first instance of entering the level, so I had not gone up the stairs from Jubilex's level and drawn her upwards).