You can use double knit just for the backing and FMQ without a problem. My guess is that it would be fine on a longarm.

I think it would depend on the knit. Some are thick and some are thinner. If it's the thick kind, you should be able to quilt a simple pattern as long as it doesn't have to go where a lot of seams meet. Good Luck!

My grandmother made all her quilts from double knit. They were all 6 inch blocks and tied. I don't have one of them - I'm OK with it - I can't stand the feel of double knit or polyester.

I'm not a fan of double knit, either, but the nursing home staff loved them because they kept the patients warm and they washed up so well. Sewn with a poly blend thread, those things were indestructable.