Shelcoof nailed (or filed it); filing the mangled corner to a nice, crisp 90-degrees resolved all the issues. See attached pictures. Note tests were only with mags containing only gas, no BBs, but I suspect all is now right.
I admit I was worried making that notch wider would lead to other problems, but I guess I had nothing to lose.
Kwokwai, check your PMs; Unicorn parts are hard to find, and I don't want to miss this chance!
Again, to all, my most humble thanks, as you not only helped me out, but proved to be a credit to this pursuit, community and forum.
See you on the fields.