A pin valve ain't good, you'll want an on-off valve on the paintball bottle. With a pin valve you'll need a seperate "isolation" valve meaning the connection on the Trevor won't be the one you're asking for.

This is an on/off valve.
http://www.ebay.com/itm/New-paintbal...-/390813702212

And no, the valve won't stop any liquid CO2 from entering the system.
The solutions used:
Positioning the bottle under an angle so the valve is above liquid level. (simplest)
An up tube , meaning there is a tube in the bottle that sticks "up" so the hole is above liquid level.
Long coiled tube or hose between bottle and reg as heat exchanger/evaporator
Buffertank