Welcome back, it’s been a while
My two cents….
The discoloration is from heat. Heat is more than likely from “bad” terminations. The branch circuit neutrals may have been cooking in the panel that was replaced, but you mention the neutral jumper buss is discolored ( loose connection)??
As to Pentrox, the Code does not mention it. It is more of a trade workmanship item, and IMHO would not be a direct cause of the heat. Think about the buss jumper.
Stay safe, and please don’t be a stranger here!