Hi Patricia!
I can't answer your question about the bus from the Studios to Downtown Disney since I've never tried to go that way. But I do know about the House of Blues and priority seatings.
HOB only does PS's for their Sunday Gospel brunch. Everything else is first come, first served. My spouse gave them a call when the Disney Dining people didn't have an answer about this, and this is what the people at the restaurant told him.
Have a wonderful trip!