1. I use the cheapest stuff I can find, where I live that's Zinsser's, both the amber and clear varities work great for me. Personally, I think the mix out of a new can is a little thick, I prefer to thin it out further with more denatured alcohol before applying it to the handlebars. Thinning it out makes it easier to coax the shellac into every nook or crevice and produces a more even coating (this is especially important if you're going to apply multiple coats).

As far as shelf-life goes - shellac has a shelf-life? A can of prepared shellac will become thick, 'muddy,' and eventually turn into a solid crust as the alcohol evaporates off over time, but you can reverse this process at will by adding more alcohol, giving the mix a stir, and allowing some time for the shellac to become fully dissolved again.

2. I wouldn't use twine on brake levers, I've played around with twine quite a bit and it's great for finishing tape or attaching cables, but it doesn't stand up to frequent handling. Even with a good tie and coating or two of shellac, it doesn't take much to displace a single loop of twine, and once one loop gets loose it's just a matter of time before the entire bundle comes apart.

Obligatory pictures of bikes whose handlebars I have shellac'ed: