I'd rather have Marnie leave Lewis. He's not healthy for her.
Maybe make Marlon a full NPC, and he gets a few events in which he befriends Marnie, and when you get to 10 hearts with the both of them, he meets with Lewis in person to have a stern talk about how he doesn't like the way Lewis treats his friend Marnie, and Marnie accidentally eavesdrops them, and she decides to finally break it for good.
Then have 4 events in the next 4 seasons in which you do some things that help Marnie heal and Marlon marries her a year after, making it the first NPC wedding in the vanilla game.
Oh, there could be a new neat feature with this: you could get to earn the wedding ring recipe even in a solo game, from Marlon, who gives it to you so you craft a ring for him so he can propose. The wedding ring could get updated so it can be merged 3 times with other rings, maintaining its appearance instead of looking like other merged rings, and Marlon would request you to put the effects of 3 random rings from a list of possible choices in the ring before you give it to him, as a cool bundle-like quest.
In multiplayer, players would be able to craft cool rings with 3 powers for their spouse, and solo players would also be able to make a wedding ring for themselves and enchant it (but they'd only be allowed to equip 1 wedding ring at a time), making the total of ring powers you can carry 5: 3 from the wedding ring, 2 from a normal combined ring.