AH HA!
Seeing as nobody has seen fit to post a plausible explanation to my situation, I have decided to resolve it by myself.
It seems it isn't so much that the aftermarket sideplate is 100% bad, it's just...different...and innocent of being the real culpret in this malfunction. The true fiend, Ladies and Gentlemen, is the TRIGGER RETURN SPRING!!!
Indeed, if I hand-cycle the action I can now fully reproduce both the malfunction, AND it's resolution! By letting up on the trigger AND pushing it forward (thanks in great deal to Georg Lugers' nearly 3/4 crescent trigger design), I can get the "snick" which brings the action back to the cocked position. Where as just letting the (weak) trigger return spring push it back into place (about 50% of it's travel it seems), the side plate lever does not have sufficiant throw to disengage and allow the weapon to complete its cocking cycle.
I actually think there may be a conspiracy at play with these two villians, and I think it is both a weak trigger return spring, along with an incorrect bevel on the (reproduction) side plate lever which are combining to perpetrate this malfunction.
I am going to replace the spring and hand file a slightly better bevel (more akin to that found on the functioning S/42), and see if it improves it. If I screw the lever up, I replace it, and am in no worse situation than I am now.
I sure wish I had made this my own thread.
It gets lonely in here...