There's a really pukka Ziploc bag called the "Zipper" which is the best of the lot. I use a fresh one every time the last shows a bit of wear. I've used a pair of them (doubled) to pack stew for a hike - so I reckon they keep the water out just fine.
Re 4). I believe it is not unknown for the gas board (or whatever they are called now) to come along and ask for their pipe back, their argument being that there are no legitimate means by which it can come into private ownership.
