Best left to a gunsmith.
Send it to the factory or a trained factory armorer
Will depend on the make of derringer. They are not all the same. Would suggest you find a gunsmith to do this one. It is cheaper than taking one to them AFTER you work on it.
Follow the instructions in the A5 manual using the link below.
You can't. Although they flex, they are considered to be one piece.
in a nice manner go to your local dealer and talk to the garage boys, if you ask them nicely they will take your stereo out. and its free happy days