Speculating on a topic like this without actually inspecting the materials is likely to be a problematic exercise, but I have an idea for you. Note well that I am discussing this having MUCH more experience with SM rigs than SN, but I do not believe that Amel made significant changes in the way they designed these systems.
I think your mizzen backstays are (much) too tight. Your description of the failure you were lucky (???) enough to have watched as involving an "explosive bang" is a pretty big clue. At rest, these wires should not be that tight.
There are a LOT of moving parts here. The length of the triatic, and the degree of rake of the masts are key. You say your rig was made by ACMO, so it's a reasonable assumption that the triatic was the right length, but you might want to confirm that.
Did you measure the tension in the mizzen backstays during the rigging process? If not measured, would you describe them as "ringing" tight?
Did you need to tension them significantly to pull the mizzen back to vertical after making the adjustments to the main mast?
Did you measure the mainmast backstay tension? I am wondering if the mizzen backstays and triatic are taking more than their fair share of the load of the forestay tension.
The fact you have had TWO failures here seems to indicate something is systemically wrong...
I hesitate to blame the swages, all the ones I have seen from ACMO where very high quality rotary hammer swages. Can you post a photo of the failed parts?
Hollywood, FL, USA.