Selling Lewandowski is a thing of the past - there is no point in talking about how good it would be if he stayed or something like that. Now Nagelsmann has to work with what he has - either change the game plan, taking into account the fact that Bavaria does not have a center forward, or endure until the winter transfer window and buy Ronaldo (this will be the only option in winter). I don't see any other options given the state of the market and the frugality of Bavaria.
Every player has their own personal opinion; once they intend and make their decisions, they will live only to suit their desires, regardless of how good or important they are to the team.
It will be advantageous for Bayern to keep Lewandowski this season as they sign Mane and other players; however, they are still good, and I hope they sign a suitable replacement for him by January.
That is true, plus he wanted to leave and when a player wants to leave then you sell them. Bayern may have faced some trouble so far but let's remember they managed to beat Barcelona, so if you beat Barcelona but lose to Augsburg then it doesn't mean that you are a bad team, you are just not jelling very well together and when the time comes you will do very well.
No team that can beat this Barcelona could be a "bad" team, they are either not having a great season because of some mistakes, or they are going to recover and I believe Bayern will recover. I think we won't even see Nagelsmann leave, we are going to probably see them keep on winning.