Hi Francesco. Regarding the child mirror, the panel is removed as follows:
1. With a thin bladed screwdriver, depress the catch in the centre of the forward side of the courtesy light. (It's recessed into the front edge) This will allow the light to be removed. Disconnect the wires and set aside.
2. Above the light there are two "pozidrive" (crosshead) screws. Remove them and the panel will be loose . Do not force it as there are also some clips to free which fasten to a floating steel frame above the headlining. Once the panel is free of the frame, it can be lowered.
3. Dependant on your model, there may be other wires to disconnect from the rear of the panel before it is completely free of the car.
Once you get the assembly out, there is a cover above the child mirror which should un-clip to allow access to the workings. If it can't be repaired I daresay you'll pick one up on ebay quite cheaply.
Replacement is a bit fiddly as you have to clip the panel into the steel frame which lies on the upper side of the head-lining, before replacing the screws.
Regarding your central arm-rest, I'm afraid I can't help with that one. I was thinking of a similar mod myself, but, like you, I haven't figured out how to remove the existing (and largely useless) armrest.
2008 2.0 TDCi Zetec