The new manager faces several disadvantages. For example, he won't be able to sign the players he wants because the transfer window has closed. He will also aim to be a tough opponent for Bayern Munich in Germany. Knowing that expectations are high, he will strive to succeed with a squad he didn't build. The new manager will have a truly significant responsibility. Appointing the Dutch manager from the start was a complete mistake. I'm sure the management has realized this by now. This mistake could cause problems for Bayer Leverkusen until the winter transfer window.
Your points are good and highlight significant challenges facing new manager. It is clear that manager who has been widely reported to be Erik ten Hag is at disadvantage because he can not bring in his own players due to transfer window closed. This is huge problem. He has been given team he did not build and as you noted team that has lost several key players from their championship winning season. This puts immense pressure on him to get results with squad that is very different from one that had so much success. Your feeling that club management made mistake with their first choice is also correct as they realized it was not working and made change very quickly. As you said this instability and poor performance will likely be problem for club until next transfer window opens.
Yes, it is true that he was given a team that he did not create, but he was given complete freedom in this regard, in fact, the players at this club could not adapt to his system. Moreover, the smartest players on this team, without whom they would not be able to generate energy, are now trying to represent themselves in other clubs at the start of the championship season. Even then, perhaps this manager would have been in demand if he had been able to continue playing aggressively from the start of this new season. It is true that the management of this club made the right move with their first choice because they understood that their dignity would not be intact under this manager, however at the present time it must be said that there is instability within this club, so perhaps the management of this club will collectively have to deal with everything in an orderly manner.