One thing that really jumped out at me was the OP stated the dragon smelled like eggs. Egg smell is usually caused by sulfur. So then I found this:
When globulin begins to decay, a toxic chemical is released that's called hydrogen sulfide, which has that very potent sulfur smell. ... Both of these proteins release that sulfurous smell, resulting in an odor that immediately tells you that this egg has gone bad.
So, it would seem there was some type of organ failure that was causing the proteins in his blood to decay causing the the sulfuric smell.
I’m no scientist, but my daughter is.
OP: Sorry about your beautiful friend. He looks like he was treated well and you did a lot of investigating to try to resolve his issue. Keep us up to date with your new scale baby.