Well, to answer your points:
1. No, I do not think that defence is automatically the problem, because you let goals in. In fact the modern day high-press was invented because it was discovered that midfield turnovers were the most common root cause of goals. Improving the defence should obviously always reduce goals against, but it may not be the most cost effective way of reducing goals against.
2. We did have a fan from another team on here and he agreed with me. He said that he thought our attacking midfielders and attackers did not contribute nearly as much to defence as other teams' do.
3. Mustafi is a poor defender in what regard? Certainly in any way that can be measured, as we can see from whoscored, he is actually one of the best defenders in England. And these statistics seem to work for other defenders everyone holds in high regard from other teams. You are claiming that the statistics work for everyone else, but have some sort of strange mathematical anomaly for Mustafi and Monreal alone? Or is it only all Arsenal players?
1. I'm not gonna argue that we couldn't improve defensively just through tactical adjustments. Drilling correct pressing of the opposition etc.. would for sure reduce the number of goals conceded. I dont dispute that.
2. Put it another way - you think any City, Bayern, Chelsea, Tottenham, United fans would be excited to see their side sign Mustafi from us? (trust me they wouldn't and most Arsenal fans would be happy to see him go)
3. Defensively poor. Like I already said 51 goals conceded is horrendous. I feel Mustafi much like Gabriel/Squillaci and Vermaelen is physically too small to play alongside someone like Koscielny.
Coming from a sports science perspective. If I were looking for defenders in this league, I would pay attention to vertical jump and height. Upper and lower body strength, explosive power and body size. (footballing qualities would come first - then physical attributes should be analysed)
Koscielny is a lightweight (75kg), relatively short ball playing CB. IMO he should be paired alongside someone bigger and more robust. I dont like the fact people pretend football is a sport where size & athletiscm doesn't play a part. It's a contact sport and size, power, speed and strength are important factors. It's all too often over the last few years we have watched Kos/Mustafi/Squillaci/Gabriel getting pushed aside and bullied on the counter by bigger more athletic centreforwards. (go back and watch the Athletico Madrid game is you want reminding of this)
On top of physical attributes there is also playing style. Both Kos and Mustafi are ball playing CB's - In that they naturally come forward with the ball and push high up the pitch. This can leave us open to counter attacks - the way to combat that would be a)signing a more traditional CB to sit alongside either Kos or Mustafi and b) having a DM sit and fall into the spaces created by the defenders coming forward. However we dont have that kind of player in the side. Xhaka's insticts are to go forward, Elneny is all over the place and Ramsey cannot sit still.
Personally I would like to see us add a quality true defensive midfielder and also get a Koulibali type CB. Someone to add a little physicality to our back line.