Hey there. Sorry about all the craziness for you. First let me say that if you really want a Maui wedding, then have a Maui wedding. St. Lucia isn't going to be all that much cheaper or easy to get to, plus, it's your wedding, not you FMIL's.
That being said, I absolutely loved my St. Lucia wedding! I'm sorry that Villa Capri ended up getting booked, Kris was wonderful to work with. She helps with several villas, so I would still contact her and ask her what options exist. Unfortunately that is the biggest private villa on the island, so you may not be able to get everyone in one villa.
Many girls have used Awesome Caribbean Weddings and some have used Down the Isle. I've heard good things about both. Technically all beaches on St. Lucia are public, but many of us get married on Pigeon Island which is mostly private. I noticed a couple or two watching our wedding from a distance, but you won't get people sunbathing next to your ceremony or anything like that.
Anyway, I would definitely encourage to go where you want to go. If you end up deciding that is St. Lucia, we're here to help! But don't let people tell you where you have to get married.