Buying a CO2 tyre inflater
Yes, this is no problem, though I would suggest practicing once before you have to use it on a flat. The first couple of times I used an inflator I had trouble inflating my tire fully.
No, but I've found that once I start a cartridge it will leak slowly, so it doesn't last forever. In my experience it doesn't last more than a few hours, certainly not more than a day.
You can buy inflators that handle both threaded and unthreaded. For example, this one -- http://www.amazon.com/gp/product/B00278XO0Q -- can take any 16g CO2 cartridge.