A lot of synthetic stocks have the pad glued on, not screwed on. You would have to cut the pad off to get inside.

If it sounds like small marbles rolling around, it' probably lead, but i would think that you could feel a weight shift with lead.

Mercury makes a sound, but it sound more like - well, a heavy liquid... kinda like motor oil moving around in a can.

I would think that the crack in your stock could be fixed with epoxy (by a compitent gunsmith) or if it is a mcmillan stock - send it back to them... i bet they'd fix it.

you're going to completely f-it up if you drill a hole in the side of it though.