So I did understand the stiffener story.
For the bolt forces, it doesn't matter if the stiffener is there.
I will explain, just run through the parts and try to draw the force lines.
Original construction:
The brake forces goes via the LCA through to bolt&nut into the chassis member.
With stiffener:
The brake forces goes via the LCA through to bolt&nut into the chassis member via stiffener to second chassis member.
==> So for both situations all the forces generated by braking go first though the bolt&nut combination.
If we interchange the chassis member by a theoretical spring. By adding the stiffener only the stiffness of that theoretical spring changes. Which minimizes the deflection, the force (still coming from the brakes) stays the same!
And then about the stiffener, we can debate on its efficiency.
Yes adding steel increases stiffness. But when you bend and create a
L or even worse a
U shape, the stiffness is only really effective along the fold.
Try at home with some paper, fold it and clamp it with your hands and apply a load
You actually added 2 folds per side. So I´m convinced the piece of the stiffener under the nut isn´t going to strip the thread, because your strip is able to elongate without bending the piece under the nut.
Oooh man what a long story, everybody still awake
Resume: I still think your bolt was simply to weak wrt the generated brake forces.