Okay, so update time:
Tested out the lens on the leica with some APX100. Thankfully, conditions were bright enough to use fast shutter speeds with f1.4, which was good as I didn't have my tripod to hand.
Sadly, the results appear pretty damning to me. The lock in these images was at infinity focus according to the RF, so all I did between these two images was take one at infinity lock, and the other slightly backwards "in front" of infinity, which according to the RF should have been out of focus.
Not the case though:
(at infinity lock)
Neither pic has been edited or sharpened, but the adjusted one is CLEARLY better focused on the lock and beyond.
I suspect the flare and softness I'm seeing elsewhere is related too, as no other examples of this lens online exhibit nearly as much softness as my copy appears to.
So it's with sadness that I must resign myself to returning this lens to the shop.
Maybe some day I'll have a working, reasonably fast and Leica compatible 50mm lens.
Thanks for the help all.
Decided not to give up so easily and added a 0.5mm copper tape layer to the metal shim in the lens. Results on the digital look very promising, with the lens hitting infinity correctly now at 1.4 and also hitting 1m as well, with maybe a touch of front focus, but not enough to throw focus right out. Will throw a test roll at it tomorrow with some real-world examples such as portraits and see how it plays out.
Also, it was really hard to see the issue on those pics I attached, for sure. :S