1. You won't be able to sew it worth a darn because it gums up the needle.
I see two possible ways to deal with this. Either sew it by hand (no thanks!), or stop every few inches and clean your needle.
I stopped about every 12-18 but you won't get as far with double layers. Major PITB.
4. Silnylon will be lighter IMHO. The little tape I used on my tarp added at least 2 ounces (~50% of the LDPE weight) as I recall.
That's entirely possible, and may even be about the same price considering the amount of duct tape I'm using, but would it be as tough? I have no experience with silnylon.
It will likely be as tough if not tougher than silnylon. However, it also won't sew as well as silnylon either since the "threads" in the tape are MUCH farther apart than normal fabric. I guess it works for cuben though, which also doesn't have a high thread density. It would have more "body" than sil though.