Sometimes soldier proof means keeping their inquisitive little digits out of the complicated bits. If it is all buried away it may not have been intended to be cleaned in the field anyway. Viz your Bren bolt bits. Only for armourers as it only needed access if something broke or wore out in which case the gunner had no way if fixing it anyway so stop them playing with it.An interesting point about Bolt/ Firing pin complexity.
Stripping down the Bolt on a Bren Gun, and remember the Taden was essentially a Bren modified for Belt-Firing, was a nightmare. And the Bren is considered to be one of the best LMG's ever constructed. Its reputation for reliability and effectiveness is noteworthy.
So much so that there were two levels of maintenance and cleaning, one done by the troops and one done by trained Armourers. A Bren's bolt had so many fiddly small parts that had to be reassembled exactly that it was beyond the ability the average soldier.
Just because a weapon has a part that is the metaphorical equivalent of a Rubik's Cube does not mean that it will not be effective or Soldier Proof.
What is evident on many of the 19th century guns I have had through my hands is that previous owners have done them no good at all by giving way to the urge to take them to pieces to see what was inside. Often followed by loosing small parts. The best were those who sat untended in some dingy and dirty military dungeon covered in filthy storage grease.