Another range day, and (mostly) excellent results. I no longer feel it's a boondoogle.
I bought a Mec-Gar 92FS mag to try out, and was not disappointed at all. It fed fine - as good as the modified Beretta* 92 mags I have. Several tests with the full mag worked without a glitch. Based on that, I will likely order more.
The Promag... I want to throw away, or more likely mark/paint as 5 rd only. It's become useless for 10 rds, always** glitching at the 5-6 round mark. It seems to have that one spot - maybe the mag was intended to be a 5 rounder that someone realized didn't need to be restricted, I don't know. If I get a chance to compare the innards with the Mec-Gar, maybe I can see where (if) there is something causing the glitch.
The issue with the rear sight moving was solved with a drift punch. No more movement.
I couldn't find the post before putting this up, but someone (apologies to whomever it was) had mentioned one of the mods he did was to trim down the orange rubber(?) internal buffer. I disassembled my carbine, and did the same. I feel it helped with clearance between the bolt-face and the ready round, but I neglected to do any before and after pics to note any differences.
My personal conclusion, to this point, is that the carbine is "not bad", certainly at the price I paid. It's got the looks, it's 9mm, uses existing magazines, and it's inexpensive (cheap even?). The stock Promag is garbage, but anyone already with, or willing to buy, Mec-Gar mags should be ok. I've found the accuracy to be pretty decent at closer ranges, with pretty tight groups if I do my part. I haven't yet shot it at 50m+, and as I have no intention of putting optics on it, that will affect the results at those longer ranges.
* these were in official Beretta marked packing, not an aftermarket item tagged for Beretta.
** as in the few times I've tested the mag. I don't have enough ammo, or money, to exhaustively test it a hundred times or so.