Here are a few pics of the mechanism. I thought again about the shutter and contacts arrangement and I have made another modification:

There are 3 leaves in the shutter: #1 closes the flash contacts, #2 opens the shutter, and #3 (held closed by a solenoid) is released after the sensor circuit has been exposed to enough light.

I cut a small piece from the first leaf which prevents it from closing the contacts. The next leaf opens the shutter and also closes the contacts, so I believe this will be the correct timing for an electronic flash (X-sync?).
(Ignore the sloppy soldering job! I did this in the early hours..)

Mechanism at rest:

First leaf about to make contact:

Modified first leaf:

No test shots yet as I need to find my piece of ground glass to calibrate the focus.