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.
2. It will be very difficult to make a pack without sewing.
3. I doubt you'd be able to glue it like cuben.
I also agree with this, wholeheartedly!
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.