One reason for the ritual is that Justin is about to go public with Harry and Elaine. They will get introduced to other wizards, learn about the laws of magic, etc. At some point Justin MUST do this as he cannot isolate himself for decades to train them in secret. He is a warden after all and has responsibilities. If Justin was black council, remember they were working towards a war 30 years in the future (during Grave Peril) which means close to 40 years in the future when Justin had Harry as apprentice. That is a VERY long time to go into hiding or retain the blind trust of an apprentice.
While it might be perfectly possible to isolate and brainwash kids to be loyal, such behavior is very hard to instill in a very independent minded and stubborn teenager who you have only worked with for 1-2 years. It might be enough to earn a significant measure of loyalty and trust, but not the level of control Justin might want if he sees Harry (or Elaine) as dangerous weapons even if he trained them for another decade as apprentices.
Keep in mind that Justin did not come across as super charismatic or loving to Harry. Also keep in mind that Harry was willing to walk away fro Eb (who he was much more fond of) in a heartbeat when he realized that Eb was acting contrary to the principles he preached. If Justin was perceptive enough to understand this aspect of Harry's character, then he knew he would not be able to effectively control Harry's behavior while openly doing bad things.
Harry walked away from Eb when he was a grown man, after he perceived him as walking away from the principles he taught him. Harry certainly loved and respected him more than Justin, because the circumstances were significantly different. Justin's sudden betrayal made Ebenezer's kindness and patience way, way more impactful.
On the flip side of things, if he understood Harry's stubborn moral code and realized he wouldn't side with him, then his approach was actually dumber. The kids sleep
in his house. He, presumably, prepares meals for them. It's as simple and straightforward as drugging dinner and enthralling them both while they're asleep, or keeping one down while he works on the other. He doesn't even need to head to the pharmacy or anything; he can brew an effective potion with Bob. The whole process could've been done painlessly and simply, which is why his approach seems so hackneyed.
It makes me think that either: A) Something changed for Justin and he had to change his plans on the fly, B) Something
about Justin changed, and he had his switch flipped from "a-hole" to "evil," or C) This is an "Evil Cannot Comprehend Good" situation and Justin thought that Harry would be okay with what went on.
I'm not necessarily saying that Lea and Harry and Elaine's interpretations of Justin's actions are definitely wrong, just that the events from Harry's perspective raise questions for me. I'd
really like to know what Justin said to Elaine before he whammied her.
