Actually, I don't think that will be the case since we're going to see a mini-Snow White in this episode. Bailee Madison was cast as a young Snow White, the Stable Boy — well, he's like in his twenties or something. So, I don't think the writers are going that route.
I agree that Snow is not a rival for “the Stable Boy’s” affections. I also do agree that Regina was forced into an arranged marriage to King Leopold (ala King George).
My theory is that Regina shares with Snow the secret that she is in love with a commoner, which as young children are prone to do, Snow (probably inadvertently) reveals the secret. I have a hunch that the commoner is killed by Regina’s mother because of this. Interestingly, this results in Regina only loving him more and spending the rest of her life trying to vindicate her loss somehow.
Ironically, this is the opposite of how King George tries to prevent James from running off with Snow (in episode 7:15) he threatens to kill James and NOT Snow because he says killing her would just make him love her more.