After nearly two and a half years, this happened:
Yup, it broke just where I expected!
The front fender on Mary’s bike suffers a bit more stress than you might expect, as she must wheel it through high grass to her Vassar Farms garden plot and the low-hanging spray flap can snag on the taller greenery.
Re-slicing the original model, printing the result, and installing it took about an hour:
Affixing the strut with duct tape and a cable tie looks déclassé, but continues to work better than anything else I’ve tried: simple, flexible, easily readjusted, totally nonfussy.
At least I now use black outdoor-rated double-stick foam tape, so life is increasingly good …
The Smell of Molten Projects in the Morning 
I’m curious why you didn’t beef up the high stress area before reprinting. It seems very unlike you.
Although I didn’t intend it this way, a weak-link bracket turns out to be much better than having something else break. Those little arches were supposed to be springy enough to let the clip bend around the fender, but PETG is much stiffer and they’re now mechanical fuses.
The rear fender bracket obviously has brick shithouse genes …