Sounds like you got first class service from Toronto Airsoft and now you're whinging about it... what did you want a whole new gun?
What you were describing is "classic" seized piston syndrome, I'm not surprised that your buddies batteries were also unable to clear the jam, often the mechbox needs to be taken apart or the anti-reversal latch disengaged to clear this malfunction.
Guh! That things shoots 440fps and you were trying to crank it with an 8.4v battery? That's definitely underpowered, but it should perform better with the stock spring installed.