The reason Quentin took Leandra was because she looked so much like her cousin, Revka Amell. The Amell girls seemed to have a thing for mages, as both would sneak out and meet with their paramours in secret. After Leandra leaves with Malcolm, her family is furious - all the Amells have left is Gamlen (a few more not worth noting, as it would seem) and their wonderful Revka, their only hope.
Revka, at some point between her family’s attempts at setting her up with someone, gets pregnant. The first thing she does when she finds out is tell Quentin, and he’s overjoyed.
And the deal with the wedding dress he put Leandra in?
The night she tells him, he proposes marriage and she accepts - they’re already planning it all - their great escape, their wedding, the names for their children.
But when Revka comes home and her family is furious - they’d noticed she’d been sneaking out and had someone follow her the past few nights. They tried to convince her Quentin was bad news but she wouldn’t listen, and confessed to being pregnant - they couldn’t keep her away from him now, could they?
Thinking fast, her uncle got as much coin as he’d need and arranged for Quentin to be transferred to Starkhaven’s Circle - he wouldn’t let another of his girls fall into the hands of a mage!
Both parties were fed a different lie - Revka was told Quentin took money from her uncle when it was offered, in exchange for leaving and never contacting her again; Quentin was told Revka said she never wanted to see him again. But neither of them believed the lies.
Revka gave birth to her wonderful child, their little Amell, and from the beginning knew her baby was destined for great things.
Quentin hadn’t gotten news of their child, and he spent every waking moment thinking about her, the family they could’ve and how he could escape to be with her again. He lost his sanity along the way. But Starkhaven’s Circle was not like the Gallows, and it took years before Quentin could finally escape (after thoroughly studying spirit and blood magic). But alas, when he arrived in Kirkwall, he found out Revka Amell was deceased - she died under circumstances no one wished to talk about, perhaps ones no one alive even knew about.
Quentin and the young Warden Amell never learn of one another.