Spacegun runs like a top! It was the RIA frame causing the problem the whole time.
After a short chat with one of our resident gunsmiths, I took advantage of a slow moment and detail stripped the frame. What I was after was the disconnector; the advice was that if the disconnect made contact with the magazine, it could be enough to hang it up. So, I carefully and slowly used a fine file to whittle away the surface that sticks out into the mag well until it was relieved and smooth as silk.
And yet it still showed the malfunction. I took it back apart again, looked over everything carefully... and determined the trigger bar was riding the magazine pretty hard, it was warped a bit. Bent it square, filed some burrs off, slid it back in... hesitation. Hrmm. Repeatably. So I removed a little bit more material at a time until it fit smoothly, no drag yet no play. Reassembled the frame, tried to replicate the problem again... and couldn't. Success!
The acid test came on the range, and the mechtech CCU performed flawlessly. Case closed. Soon though I want to re-disassemble the frame and put some manner of coating on the bits I filed to avoid rust later on.
Now, getting the thing working wasn't the big success. The big success was, I'd never detail stripped a 1911 before, and had only glanced at instructions how to about 6 hours before. What a beautiful design :) The only catch is, you can't expect a $399 version of it to run perfectly without *some* work... and now, I feel comfortable enough to venture building a new frame for the slide and barrel I have, without assistance. I will need to email Mechtech back soon, and let them know it's not their upper with the issue.