I think maybe I found the problem, & its not good news.
Look at these 2 pictures of the mechanism removed from the gun. In particular the point where the arrow points to.
I removed the assembly, washed it with gun scrubber & examined it for problems. I made zero changes to anything other than cleaning with Gun Scrubber. What I found concerns me. It seems that the reason I couldn't make an adjustment is that the rear of the set trigger is interfering with the front of the setting trigger. If I'm reading this correctly the set trigger can't go forward enough to do anything but marginally engage the secondary sear so there is no adjustment!
Once I'd removed & cleaned the mechanism the set trigger utterly refuses to set, no matter what adjustments I make, except when I manually push the triugger to the front while pulling the rear trigger to set it. Even removing the setting adjustment screw doesn't help. If I push foreward gently while pulling the rear trigger it will set, but it seems that to do this it has to push it's rear surface against the main (setting) triggers front surface! (at the point of the arrow) Before I pull the mechinism down & remove material from the rear surface to get better engagment someone please air-check me so I don't do something dumb & make things worse.
The red color is just a photographic effect there is no rust, & I did lube the mechanism after taking pictures but before trying to adjust anything.