They could have totally taken the old vampire, staked him several times, decapitated him, dismembered him poured gasoline on the parts, and then set them on fire. That's almost guaranteed to kill anything. For good measure they could have put the ashes in some sort of container and buried them somewhere, or mixed the ashes in with some concrete and used them in a building project. For people who spend a lot of time reading about vampires, they sure don't cover any of their bases.
If all of a person's scars are healed when he or she becomes a vampire, what happens to belly buttons? Do vampires have belly buttons?
Who knows. Perhaps the umbilical cord is considered to be a maternal part, so they do. Perhaps not.
How did Elijah go so insane during the relatively short period of time between You Suck and Bite Me? Did the younger vampires drive
Actually, this one's pretty easy to explain. Think of the bronze shell as a sensory deprivation chamber...with no outside influence, time gets elongated and every moment stuck inside your own head isn't a fun one.
Problem is, he escaped the bronze prison in the middle of You Suck, got time to come back to his senses and seemed relatively sane at the end of the book when reunited with his eldest "children". One just have to wonder what caused him to get all animalistic that younger vampires imprisoned him before Bite Me takes place. Blue's death, maybe?