I had the starter tested, it passed. I'm at a complete loss. Mercedes dealership might be the next step, which I'm dreading.$$$$$$
It depends on how the test was done. My starter would also have likely tested out OK.
When rebuilder took mine apart, there were bad spots on the commutator. If it happened to stop there, no start. Anywhere else, no problem. Hard to do a test that would show that. Especially if not done by an experienced auto-electric shop.
When I had your situation (and it had gone on for at least a year), I just decided rebuilding original starter on an old car was not a bad idea even if that was not the problem. By that time, I had already changed the ignition switch and the neutral switch with no success.
Fonzi's suggestion to connect a start switch that bypasses everything else would eliminate possibility of anything other than starter or solenoid being the problem. If you want to try that I am sure one of us could point you to sources for a cheap hand switch and advise where to connect it.
Solenoid is integral with starter so no point in changing just it - Just change the whole thing if you go that way.
If you go to dealer they may change starter anyway - at great expense! I once bought a new Bosch starter for my 107SL for US$99.00. Had shop install it.