@KurtinStGeorge & morriswalters
My only issue with this scenario is why Lea would view Justin as the right person to teach Harry? He was never known as a teacher - no other former apprentices are mentioned (aside from Elaine). Additionally there is strong evidence that he was walking on the left-hand path - witnessed by his keeping Bob & likely summoning HWWB to track Harry after his thralling attempt fails. Sure, Lea could’ve been wrong, but I believe Harry says in GS that Lea had always kept an eye over him even when he was younger, so wouldn’t she be keeping a very close eye on what all Justin is upto?
I think it much more likely that Justin & Simon (so Cowl) got some knowledge (perhaps because he & Maggie were ‘friendly’ or through a demon like Chauncey) that there were 2 Starborns in mundane orphanages around the country ripe for the plucking. His goal seems to have been to round them up, train them, emotionally manipulate them & then enthrall them into his personal service.
I’m also a little unsure of why Lea thinks Harry can go back & defeat Justin as a 16 year old.
It doesn't matter if Justin had or hadn't had an apprentice before. He was a wizard who both had the knowledge and the motivation to teach Harry to use his magic. Also, it's probably not a situation where Lea had several candidates to choose from.
If Lea was a passive observer and not the murderer, at some point in time she became aware of Justin's plans and made a decision not to intervene. If Lea is the killer then she met with Justin to make a deal with him. In either scenario Lea already knew Justin or knew something about him from before this time because Justin had been a one time associate of Maggie Sr. Also, remember the conversation Lea had with Harry about Justin in Ghost Story? Lea didn't know the specifics of how Harry first met and defeated HWWB, but she knew Justin DuMorne and had very specific opinions about him. This indicates Lea had enough knowledge about Justin to make a reasoned decision about his fitness to teach Harry, and probably enough knowledge to guess doing so would give Lea an opportunity to take complete control of Harry in the future.
Your last statement is a very astute. It is odd that Lea was confident that an inexperienced 16 year old Harry could defeat his teacher. Then again, we really don't know how it all went down. Could Lea have done something to hamstring Justin or even just distract him at the right moment? I'm going to have to stick a pin in that idea for now. I need to think about it some more, but I do have one thought to add. Lea wanted to know about Harry's fight with HWWB, but she didn't ask about Harry's fight with Justin. I'll bet she saw the battle from start to finish.