It simply wasn’t possible, in my opinion, to build a bomber fast enough to avoid interception by fighters but that was still capable of carrying a significant bomb-load over a sufficient range.

Defensive armament shouldn’t be judged only by the number of enemy fighters shot-down, many bombers were no doubt saved by forcing enemy fighters to shoot from greater range, from less favourable positions and with less time.

As with many aspects of air-warfare what is important is what losses you are prepared to take not what losses you wish to inflict; this is true for fighters just as it is for bombers.

That seems like a very sound conclusion. I bet the defensive fire-zone was a place Luftwaffe pilots tried to spend as little time as possible.

However, if it had truely effective, the development of long range escort fighters might not have been such a priority. They were not assigned to confront the danger from flak.