The point was that they're German, and the accusation was that certain Germans didn't have any faith in Wengers coaching abilities. So why would those 2 choose Arsenal to learn their trade at? Coaches don't earn that much money, so it can't be that.
Again, Mert is literally no where in his career, it doesn't matter if Wenger is doing a poor job or if you think he is. Reputation, you know how Fat Sam, Pardew, etc, get hired repeatedly, it's based on reputation and not much else. Walking into a job offered to you as a coach at a top tier club will overnight make him more desirable in the future and more likely to be approached for a management job than if he goes and does his coaching badges then applies to be a coach at some random club. The relationship gets exploited and he gets a leg up.
Lehman, again he may have been turned down everywhere else for a job because every knows he's ****, who knows. Our GK coaching staff is not well respected, not least since every player who left improved significantly.
You're trying to make a logically connection between A and potatoes, when they aren't remotely connected. Hell if it was actually Wenger put the idea of coaching to Mert and said hey if you do it here we'll pay for you to go on training courses. Also again, he might get paid 10k a week to be a coach at Arsenal and his other offers were £500 a week on a short term contract at Scunthorpe.
The same way actual managers often get fast tracked from absolutely nowhere to managing a top tier club, using your relationship to fast track your career is one thing and disliking how Wenger coaches is another. Hell what if Mert thinks he can simply improve the team by showing Wenger how to coach better?
Ozil supposedly had his pick of joining plenty of clubs for decent money but chose Arsenal for 350k a week... he did it for the money not because he thinks Arsenal can compete. He settled for the cash rather than anything else, lots of people take jobs they don't like because it's still their best option.