Our contractor already plans to start upstairs. There's also the staircase that needs to be done (badly!) and that's the only way to get to the upstairs.
Tomorrow morning, I will propose to him that he:
Meanwhile, we'll be living on the first floor, possibly in the library, family room, and kitchen if he's also working the living room / dining room combo. We have a full bath down here, which makes this plan survivable. The girls can sack out on the sofa beds or in sleeping bags, depending on which they prefer. Once that's done, I can move my office back out of the basement and back upstairs, because I have to be able to get to either the basement or that upstairs office to be able to work.
Then we can start coming in and out of the front door while they sand the library, family room, kitchen, and hallway.
This might work.
We'll see what he says in the morning.