Ok, to be honest, I did use once the official approach to these problems; the approach not leading to a running engine we exchanged the alternator which did work out fine. So, your approach to exchage parts from the other tenere's is a good one.
But, don't rule out the possibility of bad wiring.
As for switches, no real resistance should be measured there, but on the other hand a few Ohms won't hurt either.
An important thing is to check earthing. There is a black wire from the CDI to the battery bracket, this should be very continuous with the engine, for instance the starter motor bolt that the earth lead from the battery is connected to.
A last-ditch trick would be to bypass all protection circuits, which involves a bit of tinkering, eliminating ignition lock, stop button, sidestand switch etc. Look at the diagram and you'll see what I mean.
Succes and a good trip!