I have no advice on the Mission pedals, but do know that you do NOT want to use an instrument cable in place of a speaker.
Very different construction. Instrument cable is designed for low-voltage, with a significantly smaller wire gauges used.
Will it work? Yes. Is it safe? Nope.
A quote from Paul Rivera in the article:
Transmission of power require low resistance for low loss. Also, high current requires larger cross section of wire. Larger power needs smaller gauge-larger wire. Guitar cables are designed for low noise and capacitance, and tiny amounts of current. If you use them for a speaker cable, large losses in power transmission occur. – Paul Rivera, Sr.
To me "large losses of power" sounds a lot like heat. That power has to go somewhere.