After discovering 2 layers of old fabric and enough staples to fill a Home Depot, I decided I would not be removing the bottom 2 cushions.
I had thought I could simply cover the entire board they rest upon (it comes out of the frame with no problem) with thick batting, and then cover and staple down new fabric. Nope. That wood space down the center of the two cushions is very obvious and looks weird.
After some thought, and Googling, I have decided to give the whole seat a “tufted cushion” treatment. I went back to Jo-Ann’s fabric again (I think they know me by name now). Got another yard of fabric, 2 pieces of 2″ foam, a long upholstery needle, and nylon thread. Oh, and a button cover kit.