eave it for 10 secs and it fires anyway (out of interest is this time delay selectable/variable in SDM?)
No, you cannot make it much longer, the camera would crash (presumably due to a 'watchdog' timer in the Canon firmware).
is the next step to replace the second switch with the output from my flash trigger?
Looking at this
diagram, VOUT on CONN1 goes high at same time as flash trigger.
At present, you are using a switch to apply a positive voltage to transistor 3 base to turn it on.
Connect VOUT to the base and connect the controller ground to USB V- wire.
Setup the camera and bistable as before, press the switch to blank the screen.
Now, trigger your controller.
It should take a photo.
It will NOT be synchronised with any flash connected to the controller, we can discuss that later.
Let us know how you get on.