I believe that A350 engine control digital inputs have too high impedance. The control inputs should not change the state with coffee or other liquid spilled on/in the the switches. This applies to all reliable electronics design.
I agree, this is more complex than a couple of electrical contacts shorting out. The switch panel is likely quite complex and contains a number of processors of its own (engine fire light, fault light triggers, lighting, BITE etc). You wouldn't replace the switch if there was a fault, you'd swap out the whole thing. It's possible that the liquid is upsetting whatever microprocessor is responsible for interrogating switch position and firing it out onto the ARINC bus.
I'm surprised that there isn't backup physical cabling to the engine HP valve, but it could be that Airbus was more concerned about not being able to shut an engine down than the other way around, and implemented it through the fire pushbuttons. Certain DC bus faults on the A320 series can disable the engine master switches.