Theory:
I think the reason REGINA ends up married to King Leopold is it was an arrange marriage. After Snow’s mother dies the match is arranged to cement relationships between the kingdoms OR to get $$$$$ for the realms coffers.
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I’m starting to think maybe Regina met little Snow in the stables along with the Stable Boy, but Regina didn’t know who Little Snow was. But Little Snow is all sweet and befriends Regina, Snow runs back to her daddy and tells him about this sweet woman she met in the royal stables and Snow’s dad meets and proposes to her on the spot because Snow has taken a liking to her.
Or maybe before King Leopold proposes to Regina, he seeks out Cora and she agrees to the arranged marriage because rich = power.