When a rifle does not have an adjustable trigger, a broken sear is usually the cause of the fail-to-hold at full bent position.
Sometimes the full cock notch is rounded, or debris inside the lock prevents the sear from engaging the notch properly, but a broken or rounded sear nose is usually the problem.

As to the barrels crossing even though they are spread slightly at the muzzles, is that not a sign of too low speed in low speed guns, or too high speed in high speed guns?

Due to this being a black powder rifle ctg. by design and with black powder having more recoil (which effects regulation) at ANY speed compared to smokeless powder, a factory-type black powder charge, perhaps even with lighter bullet, might just recoil-induce the barrels to shoot more parallel. Just a thought.

I'd not shoot them anyway until repaired - but won't they have to be completely separated (taken apart, ribs and all - I'd think) before re-soldering - a nasty job and VERY expensive, I'd imagine.