Ducksie:
As someone who know who the moderators are ('cos I'm one of them!), it appears to me that the Ed usually looks for people who've been long-term members of this site and who've 'established their credentials' by contributing many answers in AB's 'serious' sections. It then helps if they also post in Chatterbank occasionally (because that's where the trolls usually appear).
Alternatively someone who has posted mainly in CB for years, without ever offending anyone, might also be considered as a moderator.
Someone who was previously been trolling the site would need to exhibit good behaviour (under their new username) for a considerable period before they could be awarded moderator status. As most trolls can't even go a day without posting something offensive, I doubt that they could manage to be positive and helpful for several years just in the hope of becoming a moderator!