I too would suggest cleaning all terminals, grounds, etc. Both big battery terminals. The ground strap from the battery to the chassis. the ground strap from the transmission to the chassis. The fat yellow wire in the 14-pin connector at the fuse panel located in the engine compartment. Then do the load testing mentioned above. Yes, it could be your starter but I still have my doubts.

Yes, you hug the transmission in order to reach the top nut. It's right there when you reach around. Be certain your negative strap is disconnected from the battery. That fat red wire going to the starter is UNFUSED and always hot.

Gunter,
I haven't been able to measure the voltage at the battery or alternator wires yet as i can't find my volt meter. Tried the jumper wire (used a remote start switch) and yes, nothing.

The battery is a brand new, fully slow-charged Interstate, of the proper amp-hrs and cca. I'll try the direct battery-to-starter rout this weekend. I do hear a click of some kind, when I turn the starter switch in the cockpit, though.

I guess that the two issues which bother me the most are that this SC has been trouble-free for over 8 years no; until I parked it to change out a F to R oil cooler line; and that I've not found a real independent Porsche shop in town since I moved here 2 years ago, the dealer is a joke for anything older that 3 years... whine, moan, poor me, etc.

But I'm going to hit this problem hard this weekend, it's seeming more and more like it's the starter/solenoid, from all this wonderful advice I'm receiving here, but I'll try and eliminate all other possibilities as best I can.
A thousand thanks for all the support, from everyone.
Andy