Many thanks for your thoughtful and experienced reply plus your sage observation about speculating from afar - that's always an issue with this DB, but the idea flow is super, regardless.
I did an autopsy on the failed swage today - the first picture shows the inside of the swage with the outside strands having broken right at the top and the core strands breaking maybe an 1/8" down into the swage. The second picture, where I've cut the swage open, seems to show that the cable further down in the swage looks perfectly ok. I think the brown-ish surface stain down deep is just the grease they apply during the swaging process.
Below is a picture of one of the other stays which is showing pretty much the same rust as the backstay that broke. The picture on the right is of the end of the broken stay. It shows that all the outer strands broke at about the same level and the core strands are a little longer, that is, they broke a few mm's into the swage. (The perspective make it difficult to see that they stick out a bit, but they do.)
I have been in touch with ACMO and they said that, while ss does not normally rust, it is important to thoroughly wash the wire above the swages with soap and water when they are new to remove the swaging grease that may migrate out of the swage and form a dirt trap that can cause rusting. The actual reply, which may lose a bit with the "Franglish" was,
" It is important to know that this is not necessarily rust, the parts are in stainless steel and cannot rust except for concerns of contamination with an external element.
Did you try to clean with soap and water?
But if it's broke, it's broke.
For the futur : The first months, it is important to clean the cables with soap and water because grease can come out and "stick" the dust, which gives this rusty color.
The products used for the manufacture of cables are more respectful of nature and therefore less powerful than at the time :)"
I'm not quite sure what that means, but it may be sage advice to clean new swaged fittings early on.
As to tensioning, no, the backstays were not "AMEL tight". I keep the main mast stays "rock hard" but not the mizzen back stays, which, while "quite tight" (that is the leeward stay does not sag on a windy beam reach) , would give 2-3 inches when you grabbed them. I did not use a tensioning gauge when I set up the rig (for the larger diameter wire a tension gauge is hard to come by). I used Olivier's and Seldon Mast Co.'s recommendations (measuring the elongation of the wire as it is tensioned to get to about 15% of breaking strength), plus fine tuning at sea under ideal conditions (flat water and 15-20 knots of breeze). I do keep the other mizzen stays "Amel hard".
The earlier failure (in 2009), which was of the forward mizzen lower was virtually identical in appearance (external strands broken at the top of the swage and core strands at 3-4 mm's down), except that the stay was under much tension as we were power reaching at a screaming 8.5 kts in a 25 kt breeze. That was the original 1992 rigging, so it was 17 or 18 years old. We often thought it may have been due to that stay being the one we always grabbed coming up the ladder after swimming in salt water and the water would drip down into the top of the swage. Perhaps.
I do not think the cause of this failure can be definitively stated. However, I suspect that 5 to 6 years in tropical Florida waters and bobbing up and down with "cursed stink boats" going up and down the ICW at speed" presented a "work hardening" effect on the back stay, made worse, actually, because it was not "Amel tight". Plus, frankly, we did not religiously clean the stays above the swages, as ACMO recommends.
As of now I plan to replace all the rigging and use mechanical fasteners throughout (except for the "special" lower headstay termination). Amel is sending me a quote and I'm getting quotes from Nance and Underwood and Florida Rigging. I'll update this post as I get more information.
SN68 Sangaris, Tropic Isle Harbor, FL