Here are some updates.
I had troubles with monopoint, and after changing all parts possible it seems that wiring was a problem.

Now all parts except injector and computer are new - mostly genuine spare parts used.
Since car had idle problems, I started investigating by changing stepper motor. I got a new one made by Meat&Doria - altough their catalogue states that this stepper is for BX 16i, it turned out that it doesn't fit BX at all.
Monopoint body on the table:
And wrong stepper motor - it looks the same as one on BX, but the connector is rotated in 90 degrees to the left.
Since this attempt of using non-genuine spare part failed, I got a new one, genuine Magneti Marelli stepper motor form Citroen dealer. Instead of using bolts that came with it, I changed them with universal hexagon bolts - easier to remove the stepper when monopoint body is on the car.
Left to right - bolts I used as replacement, genuine ones that came with the stepper and bolts removed from the car:
Since changing stepper didn;t help, I got throttle potentiometer and changed it too - got 70% discount at local Peugeot dealer, since they are cleaning their od car parts stock. Also used hexagon bolts.
On the left is Solex potentiometer that I removed from the car, and on the left is Magneti Marelli that I got from Peugeot.
There was still no change, I still had idling problems, until one cold day when everything came back to normal.

There was probably some bad contact problem, will have to check all the wiring ASAP.