Can I use packing popcorn (peanuts) instead of gravel for filling a french drain?
I don't think packing popcorn will work. I'd think it would compress and leave at best a spongy spot in your yard. Most likely you'll end up with it collapsing as soon as any weight is applied over it. The material shown in the two products you listed is a polystyrene aggregate. I don't think packing peanuts are the same exact material.