Good to see that you are making progress.
My Zorki 6 and I had quite a time over a few weeks when I stripped it down to re-glue one of the curtains. There's lots of things to set up that can catch the unwary, the gap of the shields is one such thing. (stripping the thread on the wind lever collar or damaging the wind ratchet is sooooo easy, but that's another story.)
My FED2 has replacement curtains and can give problems with light here, this is also because the set up of the curtain is wrong, and possibly by only 1/2mm, ouch!)
You could check out the curtain-shield gaps and think about moving the shields if necessary, but to be honest, if there were no other reason to take the top off, I would be very tempted to bend the shield into place. If it fails or gives more problems, they are easy to bend back.
The thickest "thing" to pass between the curtains and shields is the two laths when cocking. In this case they overlap. This overlap does vary and is critical for leaks of this type. You have identified that the curtains move at different rates when being cocked. This is because one curtain is being wound ONTO its drum, so is increasing in diameter as cocked, so progressively pulling more curtain as winding continues. The other curtain, as you can guess, is being unwound OFF it's drum so the rate of curtain "feed" decreases as cocking continues. This is progressive across the whole film gate. I would say that my Z6 (the same as any other of it's type) has a difference of around 3mm over the full movement.
So, back to the shields, you could gently check the clearance between the top and bottom shields as the curtains are cocked (doubled laths). You only need enough clearance so as not to make the curtains bind when cocking. When shooting of course, only one lath at a time will pass through the gap.
The overlap: This can easily be wrong and is the most critical factor in replacing curtains.
A good set up should start the cocking with very near to 1/2 the bottom lath overlapping the top. By the time it finishes it will be fully overlapped. Check too that they are parallel.
If this is far out, you have much work to do.
Matching laths: By this I mean that the laths should lie on top of each other without a gap between them. It's very easy and quite common for someone to have poked one lath and bent or bowed it out of shape. Half way through cocking, check that they sit well together, bend both to try to get a matching "bow".
Hope some of this helps:
Dave..
