Answer to # 3 - Yes, your husband is a sweetie!
If you buy a home with a large family or a living/dining room 'great room', then that would be a good choice for sewing in, since DH wants to spend time with you. But remind him that sewing is not only messy, but can be noisy, too!
Do you ever plan on getting a long arm? If so, the bigger your space the better. We have a house with 5 garages, 3 of them heated and, most impressive, the 3 car garage has a full bathroom as well. That is why we bought the house - DH uses the 3 car garage for his hobbies; he very kindly offered me one bay for my HQ16 on a quilting frame, and I accepted. (This happened AFTER I threw him and his overwhelming hobby collections out of the house! What a guy!)
We spend lots of time together doing our own thing in his garage. However, I do all my piecing in the house in the dining area of our great room, so the noise does interfere with his TV. (Most of the time he falls asleep in front of the TV so it's not that big a deal.) Bottom line is that sharing a space sounds wonderful, but may not live up to your expectations, so think about it carefully.
Question #2 - my sewing areas are on the main floor. My DH uses the bonus room that I thought I'd use for his music. I'm glad we decided to do this, as climbing stairs is getting harder for me as time goes on. We also use the bonus room as a guest room, so it does double duty, especially when the 'grands' come out.
Also, if you frequently have 'grands' over like I do, and they like to sew, it's much better to have your sewing area in a place that is easily supervised. If you have 'grands' over a lot and they have no interest in sewing, then you could sew in a more isolated part of the house - like a bedroom or a loft.