Ken,
I'm not sure about the "S" wire terminal. What I found was 2 ring terminals on one post (pretty sure one was battery power, thicker red cable) and then a flat terminal connector on another one (maybe "S" ?) both facing forward and somewhat under the alternator. I am assuming that was the solenoid of the starter. Admittedly, multiple assumptions being made here. In any event I cleaned the rings and unseated\re-seated the flat.
I have a separate key and start button. I hold the key in the spring loaded temp position for 30 secs then back off to the start position, then hit the start button.
Understood that the solenoid does nothing to help starting problems and I agree that you are just buying yourself 20 seconds. I also get the fact that you are introducing more failure points. The potential of a low battery is not a huge concern as I do have a portable start battery with jumpers aboard and we rarely use any juice (no overnights yet, no refrig, no inverter). My logic, FWIW, is that if the engine is cold and we are sailing there are times when I need the engine to get me out of a bit of trouble - stalled on a tack or needing to give way fast in close quarters, etc. Then, if it doesn't start, repeating the same sequence (thinking i didn't wait on the glow plugs long enough). So, I guess I was looking at it as a fairly easy and cheap upgrade for a pressure relief. More psychological than anything. Also, makes it easier for the admiral to start her if needed.
I do already have the instrument panel with the voltmeter.
Of course I saw your relay idea, in the tech wiki, after I purchased the solenoid.

Actually, I saw it before, but it makes more sense now that you walked me through it. Maybe I will rethink my approach now.
One question while I have your attention. After holding the key in the temp position, and consistently at about the 25 sec mark, there is an audible click at the instrument panel itself, almost like a notification that the plugs are warmed up. I haven't read anything about this. Is this anything that you are aware of? I'm definitely not imagining it.
Thanks,
Bryan