Simple: Each early bird takes a window or aisle mid-cabin (past row 7).and waits for the others to arrive. Nobody's going to argue with your saving a middle seat.
Yup. Make sure it's past the first few rows. If you run into someone like me and I'm in a hurry, I will pick an upfront middle seat so that I can make a quick exit. So don't be saving a middle seat in row 3. I will just take it without asking if the forward lavatory light is not lit.