Have you tried taking the main reg of the gun, with stuck bottle, and then removing the melee from the bottle valve? Use a Boa as suggested on the bottle and with some help, a big f~ck off wrench around the flat bits on the melee.
Is all the air out of the tank, I know you said its empty, but could theer be some residual preassure, not enough to cycle the marker, but enough to keep the bottle tight?
failing all others. call John on the number in my sig, he assembles the Melee regs in the UK, he might fix it for you, probably just for postage, he's nice like that, or go see him if your near Essex.
If the regs are knackerd, get the bottle off anyway coz thats the expensive bit!
Gaz