So you have checked that power is getting to the contact on the cut-off
solenoid. Have you checked that the solenoid is actually working? They do
fail, either electrically, or stick mechanically.
There is no immobiliser which will cut the power after you crank the
starter.
If I remember correctly, the central locking on my '93 405 would also
immobilise the starter. Unless you have some aftermarket system fitted.
If you have fuel getting into the pump, but not out, how have you checked
this.
It sounds as if either the cut-off is stuck closed, or you have air in the
system.
To get the air out, you need to crack open the high pressure connectors on
the top of two injectors. Then crank the engine with the accellerator pedal
fully depressed.
Keep cranking until you get fuel spurting from the connectors. You will
probably see bubbles first.
As soon as you get fuel, close those two and open the other two, and repeat.
Then the engine should start normally, unless you have a leak allowing air
to continually enter the fuel system.