The idea of keeping the middle seat empty is the topic of endless discussion and controversy on TripAdvisor. Those who practice this generally do so with the intent to offer a choice of the aisle or window seat to the middle passenger if one shows up. But there a few who flatly state that they must stay in their assigned seat and would refuse to move. I do not understand their reasoning at all, nor do I agree with those who argue it is rude to claim the aisle and window seats in hopes of keeping the middle empty.
We use miles to fly business class on long haul flights. But for domestic flights, after four successive flights where my middle-seat husband was nearly squashed by the aisle passenger, too large to fit in his own seat, we started booking aisle and window. We choose seats far to the back of the plane, on the theory that middle seats there get filled last. If a middle seat passenger does show up ( and this has happened only once) my husband stands up as he would to let the middle passenger in, but immediately asks if he or she would like the aisle seat instead. Acceptance was immediate.
As for meals when you have swapped seats, you are seated close enough that you can just advise the flight attendant, or sort it out yourselves.