Shanks straight up had the life extinguished of the guy pointing the gun at him. You are right that he doesn't get upset easily. He can take all the shit thrown at him and not be bothered by it. But that's part of his positive outward image. That's what machiavellism is all about. Which is the defining characteristic of Shanks. Shanks needs to be the good guy of the narrative in everyone's minds. And for that end, he needs to sell himself as the kind of person that everyone will look up to.
Anything you see Shanks involved in is not necessarily actually happening. Saving Luffy, saving Buggy, getting anything spilled on him. He is a lunarian and the user of the Illusion fruit after all.
You think because Oda makes mistakes, he will just accept the mistakes and move forward with them? Do you not see what Oda meant by that statement? Errors are meant to be corrected. He needs people to keep reading until he can make the shocking reveal that will explain perfectly why Shanks 'lost' that arm. Does it make sense for a story writer to finish with plot threads being loose or where the readers will be left confused or disappointed, with the explanation given being 'oops!'? If a story writer is self-aware of a mistake they made, they will try to change parts of the story, so that this 'error' makes somewhat sense and the audience can be at peace with what was written, while he also tries to not mess up what he had already planned out through these corrections.
Teach avoids to fight Shanks because Teach is level headed. He does not fight battles that he knows he can lose. He only goes for straight up victories. He does not fight equal matches and he does not fight losses. When Shanks was a kid and weaker than blackbeard, he could have taken Shanks out but there was no point to it. Teach knew that Shanks is not a threat to him in the long run. Instead he aimed for Luffy, easy win to establish himself, and one of the D and thus a threat in the future. And even after he got hold of two of the strongest devil fruits, he still admitted that he was not keen on fighting Shanks yet because he knows that he would be going to lose at that time.
In short, Teach avoids battles he knows he will lose or where the gains are not sufficient and he picks fights where victory is decided beforehand and gains can be made from it.
The reason shanks takes the One Piece very seriously is because as a lunarian with knowledge of the void century, he knows what it is and it is directly related to him.
I can see why people would have reason to think that the claim of Shanks being a lunarian and user of the illusion fruit may seem to lack foundation to them. A lot of things are happening fast now. As with many manga who are about to end, the pace is picking up and not everything is equally clear to see for everyone. That Aokiji is now a member of the blackbeard pirates. Or that shanks is a lunarian and he is going to fight luffy and the other 'good guys', just like Roger who allied with the marines.
Just like Oda whose main audience are the new generations, Shanks is a character that grooms the children for the future. He sets them up, for destruction. The lunarians are a very obvious allusion to the fallen angels, who came crashing down 'like lightning' to Earth.