The bomb sight was not used to aim the rear guns.
The periscope could be rotated 180 to face forward. When in that position it was used as a 'bomb sight' for glide bomb attacks (shallow dives from altitude). If you can't comprehend that then just stop posting.
The rearward firing 2cm were almost never fitted and were completely useless, periscope or no periscope. They are as equally useless in AH. However, having the periscope modeled so that it could be used as a 'glide bomb sight' would be much more 'useful' for high speed, higher altitude bomb attacks. In real life the 234, as a bomber, didn't achieve much. It was a a recce aircraft that it proved its worth.
Also from the link you provided:
BZA1-B Bombing Computer
What do you think that means?