Oskar, Thank you for your reply. I had success times two but before that let me address your question. The only twisting was incidental. Nothing at all like described in edit 1 which I did not see until after doing the repair this morning. The strobe was mounted on a ball at the top of the housing (ball normally used for pole cam or focusing lights). So the only movement came from plugging and unplugging the plastic end into the strobe and incidental movement to move the cabling out of the way - I used a standard curly Seacam S6 - S6 cable. After the initial failure I started swapping strobes so there was a little more movement of the cable. First the old model. When it failed I got a Seacam S6 - S&S cable which I attached to the opposite bulkhead to confirm functionality of strobe and camera which turned out OK. Then I went for the second adapter - at this point I had not yet opened the box containing the 2nd strobe plus accessories. Number 2 worked OK as I described yesterday.
Today. I first fixed the problematic adapter. It was challenging to remove the broken bit. I found success by using a micro screw driver to wedge it out about 0.5mm. I was then able to grab hold of it with a small needle-nose pliers - the one I had in my UWP tool kit. The transparent part came out first. Following a little more digging with the screwdriver I got enough of the black part out to grab with the pliers. The screwdriver I used as well as the two parts of the cable that I removed are in the attached pic. One can see the wear on the ends resulting from my extraction technique.
I then attached it to my housing after mounting the old strobe. Made sure it was on SL and it worked! I think I had it on to just "on" yesterday. So success on both the cord as well as the old strobe working via fiber optics.
My suggestion is to have the hole at metal end of the adapter part be of dimensions compatible with the plug that is on the strobe end of fiber optic and use the same plastic part on each end. Think double-ender like a Viking ship. One could unplug the fiber optic part from the adapter part for traveling making them easier to pack. Fiber optic cords could be packed separately into a more or less flat compartment. This would reduce stress on the fiber optic part since it would not have a heavy object attached to the end (the adapter part). People like me would be happy if you sold the plastic plugs by the boxload (2 dozen or even 50!). I like having backups plus the cords could be made for different lengths. The set screws could even go in a small ziplock bag saving the labor of installing them.