Very tenuously supported. Two points. First Eb had met Malcolm, says he was gentle. And he attended a meeting with Margaret discussing a plan. Margaret breaks with Raith and bails, two years or so she has Harry and dies. Eb never says where he saw Malcolm and it's never revealed how Margaret broke away.
Eb met Malcolm at that same meeting. The meeting was about producing a child as a Starborn. Malcolm is a scion of an angel. Eb objects but Margaret says to hell with it since she is under the influence of Raith and has the baby anyway and falls for Malcolm releasing her from Raith's control. True love. Possibly Malcolm was himself controlled by Raith.
Pregnant and having to scoot, she leaves Thomas but not before connecting Thomas and an unborn Harry so that when she throws her death curse it can be powered by them instead of her life force. She goes into deep Winter under Lea's protection and loses a year while Harry comes to term. And leaves Winter via a portal in the Lincoln Monument. She gets the protection for Harry by telling Lea who has the Atheme. Lea sells out Malcolm to the Whites when Harry is six. And just to make it a real pot boiler, Lara does the deed and leaves Malcolm smiling.
It reads like a cheesy potboiler and I don't know if it is reasonable or not. But it connects the dots without tearing the fabric too badly. Feel free to rip it a new one. 48 days to Peace Talks