I really enjoy exploring such things. I did a short research on this topic. I was killing Diremaws and dropping their bodies into another location where they don't normally respawn.
I then observed the corpses. It turns out that Diremaws do in fact respawn from dead bodies.
During my research I gathered enough information to answer all your questions.
Can they spawn from the corpse at any time or only when the corpse changes decay stages?
Diremaw can spawn when the body changes state of decay. Here's the proof:
If so can they spawn as the corpse disappears?
As you can see in the picture above the body does not disappear but changes its state during the spawn.
What is the % chance for a Diremaw to spawn from the corpse and it it affected by the corpse decay stage?
I killed 62 Diremaws during the reasearch. Six of these monsters were spawned from the bodies. It comes out to a 10,71 percent chance that Diremaw will spawn from the corpse. I suspect it may be exactly 10 percent, but to confirm this would require killing a large number of these monsters.
Can multiple Diremaw spawn from the same corpse?
I have not observed this.
Will they only spawn if someone is on the screen or can they spawn at any time?
They can spawn at any time. Also when the player is in a completely different location.