No, you will not be allowed.
Yes. Nitrogen, or High Pressure Air as its called Is a much better alternative to Co2, because of its better consistency and the fact it never enters a liquid form.
No, but they must use a propellant. The only other choice is High Pressure Air also know as: HPA, Air, Nitro or Nitrogen.
No. There are no urban areas that allow paintball.
No, any marker that says HPA only means HPA only! co2 will damage the marker