I would sell them--many people are interested in Mid-century Modern furniture--and use the proceeds to buy weather-resistant furniture for your deck. These are too wonderfully designed to take a chance on ruining.

Regardless of what fabric(s) you use, the frame and fastenings won't stand up to prolonged outdoor exposure. I'd find room for them inside, store them until you do have room for them, or sell them.

There truly is no space for them indoors. You're looking at the largest room in the house (photo above) and none of the other rooms have the space for seating of any sort.

Since Herman Miller is local around here there are quite a lot of these sectionals around and sadly, they're a dime a dozen. Selling them would likely not bring in enough to even purchase a single piece of outdoor furniture.....even if I could find outdoor furniture that I like.

These chairs consist of a plastic base and a shaped urethane foam body. Why would that not hold up to a covered porch area (as large & deep as the livingroom) if appropriately upholstered and put into storage for the winter?

They should be fine as long as they don’t get wet and the covered porch should protect them. Go for it!