OK, here's what I did.
1) Assemble with side plate, but without the breechblock assembly.
- Finger off trigger, push barrel forwards, then squeeze trigger. Sear bar spring is being worked as expected.
- Pull barrel back, squeeze trigger all the way, then push barrel forwards. Releasing trigger slightly and re-squeezeing does not work the sear bar spring, as expected. If I release the trigger all the way, I hear a faint click, and the next trigger pull works the sear bar spring.
- Pull barrel back, squeeze trigger all the way, then push barrel forwards. Release trigger gradually until I hear a faint click - this does not happen until the trigger is almost all the way released, the click is just about when I'm taking my finger off the trigger.
2) Assemble as normal but without the side plate. Pulling toggle back and releasing, I can release the firing pin every time by pushing the sear bar inwards on the plunger side.
3) Assemble normally (with trigger spring, I did not take this out), hold trigger fowards, work toggle action. Nothing unusual happens, the firing pin does not release, and if I remove my finger and then squeeze the trigger, it behaves normally.
4) Assemble normally, squeeze trigger slightly, keep squeezing and work the toggle action. AHA!!! If I now squeeze the trigger more, the only resistance is from the trigger spring, AND if I then take my finger off and then squeeze again, it does not engage the sear bar spring, just mush. If I release again, either I hear a faint click and the next pull is normal, OR it returns to the previous state, in which case I have to cycle the trigger at least one more time before I hear the faint click and the next trigger pull is normal and releases the firing pin. This is exactly what was happening at the range, with ammo, so I can reproduce it in the garage by working the action while keeping slighty pressure on the trigger.
5) Remove side plate, put a piece of tape on the underside (down) of the lower arm of the trigger lever, then reassemble normally and repeat (4). No problems, if I keep the trigger squeezed while cyclign the toggle then it might be mushy to squeeze it more, but if I take my finger off it clicks and then fires normally every time.
6) Take the piece of tape off and repeat (4). Exactly the same thing.
So the tape fixes it. Obviously a temporary kluge, but it does point to where the problem and solution are. The trigger lever rocks a bit in the side plate, forwards/backwards. I'm wondering if holding the trigger rocks the lever enough to cause it to hang on the plunger, keeping it from springing out even after my finger comes off?
Either way, the issue seems to be in the trigger lever/plunger area, and huge kudos to tangolima for guiding me this far via PM! I'll attach the pics you were looking for below - I could not get a pic with the plunger depressed, I don't have enough hands, but when fully depressed it seems basically totally inside the hole in the sear bar.