How would Thomas know that Justin was involved? That's my only reason the theory that Thomas was warning about DuMorne can't be correct. I also think it's more likely that Justine is betraying him and Lara, and Thomas was warning about that.
I'd really like it if Maggie had talent, and we got to see a short story or a couple of chapters in a book about her and Harry hanging out with River Shoulders one summer for training.
I'm not saying it's likely, but here's the scenario.
1) Margaret and Justin were associates, and both attended a party at Raith's with Eb about a scheme they cooked up.
2) There's no reason to think Raith and Justin fell out after Margaret left. They might not have been friends, but Justin might have come around. And Thomas might have been old enough to know who he was.
3) Justin lived (the longest shot of all of these).
4) Justin showed up and enthralled Thomas to have him attack Etri, hoping he'd succeed and die in the process to cause chaos and division and screw with Harry.
5) Thomas recognized him but couldn't stop the enthrallment, but was free after the command was finished, and tried to warn Harry.