Others have pointed out the impracticalities, but if you're determined to use a blender motor post the spec of the motor (voltage/current/power/frequency ratings).
Welcome to EP!
You're looking for a 'seat occupancy sensor'. But the type used in a car seat is expensive.!
A coil spring (as Minder suggested) or leaf spring, operating a lever-actuated microswitch, would be my choice.
Not knowing how the camera has been housed and used, is it possible salt water has penetrated the female connector at some time and corroded the contacts?
The schematic indicates that leaving the strobe disconnected won't affect the camera triggering.
Check with your meter that pins 1 and 3 show permanent connectivity with the cable connected.
Nothing else. Just bridging the two contacts on that lower plug with a piece of wire should do it. If it doesn't, then it looks like there's a problem with the camera [Edit: or its connector].
Providing all conductors in the cable are intact (have you checked with a meter?), and the contacts on the plugs and sockets are clean, then shorting the contacts that go to pins 3 and 4 should trigger the camera.
The stored energy in the cap is not enough to heat the low-side output FET of the inverter significantly during the very brief discharge time.
Yes.
Good to know the circuit works for you.
Looks as though the solder paste has crept under the edges of the mask apertures. What hold-down method was used to keep the mask closely in contact with the board?
No. When the diode is reverse biased it stops the cap discharging.
Yes. Since the cap is only 10nF the current won't hurt the inverter. If a much larger cap were used (say 100uF or more) a current-limiting resistor might be needed.