Hi,
Jug, you are asking a lot without really describing first what sim you are using. If you are using CFS1, no plane has automatic rear firing guns as it was not built into the program. The same for CFS2. The trick we use in designing guns for the rear is to allocate them to a cannon trigger. Try downloading my Stuka or Me110 fromCFS2 downloads and use your cannon button and see your rear gunner shooting, however, he wont shoot on his own!

As for the panel showing up black, that is a fault in the design of the specific panel by the original designer. You would have to send the file to a designer to rectify the problem as it can be a little complex.
AS for the problem of CFS2 wingmen not hitting anything, they can be changed in the missions from novice to Aces, however this function isnt built into quick combat. You cannot adjust their shooting ability either.
As for the Do335 panel, I suggest you substitute the Fw190a panel for it. Just copy the stock(cfs1) panel folder and add it to the D0335 folder replacing the panel. At least you will be able to see when your engines are dying.
As for adding guns, DPEDIT version 110 does it well, but you need a basic understanding of how the file works before you can effectively add guns.
All of this kind of stuff takes years to master(I know, I have been doing it for over 5 years and I am still learning), so dont get frustrated. Start with the basics and learn how to repaint a plane in CFS1(the easiest), then learn about damage profiles, then guages. Who knows, you might even end up building your own planes in GMAX or FSDS one day!

Good luck, remember patience is the key
Happy Hunting
DieStaffel