Disclaimer, I am not a SMR expert, but I have 400+ balls through it.
I had the same trigger problem after 350 or so shots.. On a guess, I moved the screw until it did not set, then backed it off some (?) amount of turns. That seemed to fix it. I had no issues since then, maybe 50 rounds? Like you said it is a mystery, but somehow after it works again.
My flash hole area gets caked up in 15 rounds or so. I use 4F. At that dirt point I could have a flash and no bang. So I clean the flash hole area with a tooth brush, and poke the vent with a paper clip. That fixes it. I think the vent is a little low, but again I lack expertise. My flash pan is polished to help with cleaning.
For years I had misfires due to light swabbing cleaning. I quit swabbing, and reliability increased dramatically. I don't swab now, but I do blow down the barrel after every shot.
I had williams adjustable peeps drilled and tapped on the barrel. Factory sights were not really considered. I simple can't seem the factory sights well.
As far as powder, mine has a auto close valve, but I put it on a table behind the shooting bench.
I shoot 40 grains 3F goex for practice, and 70 grains to hunt whitetail.
I'll add 445 ball was better than 440. I use 445 and 0.015 prelubed patch.
My SMR is very reliable except for dirty flash hole, and the quirky trigger. I actually thought of calling Kibler, but somehow I have it working again. I wish i could get the set trigger to be heavier, it is pretty light for cold winter shooting. I have a single stage trigger on the woodsrunner, and it is probably 2 pounds and really great.
I run 5 or so patches with simple green and water followed by moosemilk patches at the range. I do the same at home, followed by a few balistrol patches. I toothbrush the lock with simple green and hot water.
I think my SMR is a crazy nice rifle, but it has the same "issues" you have stated.