Just a couple of thoughts to throw out there for consideration, in case they didn't cross your mind. You have lots that you are thinking about with this trip!

1. Don't forget to factor in tolls to the cost of the rental car. It's not a huge cost, but just something to factor in.
2. The buses from SSR aren't any worse than any place else. Sometimes they are great, sometimes they are not. I've had this same experience at every WDW
DVC I've stayed at where I had to take a bus.
3. Being at SSR will put you close to DS, so if your BIL and his wife end up not liking the parks, they can always walk over there and walk around and grab a bite to eat over there.
4. I would be bummed about giving up my BWV location too, but the renovated SSR rooms are quite nice. With a 2br, you will feel like you are in a mansion compared to the BWV studio. LOL!!!
I hope y'all have a great trip!