The Stop Solenoid activates a small push/pull rod on the back of the injection pump that when pulled out somewhat, cuts of the fuel. The injection pump on the 4-108 is on the port side of the engine (in our case) directly below the heat exchanger and can be a little awkward to reach but one needs only two fingers to move the rod an inch or so. If you are worried about the need, I suggest you try it once of twice so that you know how to do it when it might become necessary.
I'm having intermittent problems with mine. It's a Perkins 4-154 I tied a small line to the shutdown lever the solenoid pulls. When the electric doesn't work I can just open the engine room door where I tied the other end of the line and pull. This is/was a end of season "fix". My boat still has the pull cable fitting on the pedestal. I plan to go back to a cable system. Don't see a need to make a simple system complicated by making it electrical.
I did use a plate over the intake the first time. I wouldn't use my hand, bad idea not because of back fire but blood blister from the vacuum. I had heavy equipment buddys at Ferris State University tell me about a runaway diesel. It ate a 3/4 plywood board the used to try to stop it.
I'm about to install a shut-down solenoid on my 4-107 - only because it has to be there to auto-shutdown the engine for the new FireBoy fixed fire extinguishing system that I just installed.
That being said, I will be keeping the original manual pull-handle shut-down (it's a Morse cable system) for safety's sake. Both shut-downs can operate in parallel using slotted hole connections at the shut down lever.
As has been mentioned, the intake mushroom on your engine does not make it easy to kill the air. I would suggest that you add a Morse cable manual shut down - better safe than sorry.