Hi, Scott!
All those plants you mention don't really need a poly to over-winter, parella is a good winter lettuce.
You could do peas in there and more delicate salads, some oriental veg and over-winter your touchy things and start off things early or leave long maturing ones in there for late harvest. Could start off late fruiting toms from cuttings for example.
Maggots in peppers can be a couple of things, once in there though, nothing you can do. I find no incidence under cover.