"The importance of DMs sometimes retaining sole authorship"
The thing is though, you aren't demonstrating that. There was nothing wrong with what your player did, you just don't like it an find it awkward because you have to tell them no. Your players don't like it for reasons that frankly, you aren't expressing well, except that they seem to also be upset someone wrote something into your world.
I think I expressed it rather well actually.
The player did nothing wrong, and there is, from a practical standpoint, nothing stopping you and your other players from embracing his additions, except for the fact that you don't want to. Sure, you don't want to do it, and it is rooted in some instincts I respect, but there is no importance to deciding one way or the other.
I think he did. This was not something we agreed on. You don't just prepare a portion of another DM's campaign. That is rather rude. At the very least, a player should ask first.
Permission? Why did he need permission to write-up something and present it to you? And frankly, if you had liked his ideas, since you are the "sole author" then you would have implemented them whether the other players liked them or not.
I'm not sure if I would have. But yes, he needed permission. Absolutely. He didn't just prepare one or two characters. He prepared a handful of them, all with their own backstories. And he just threw it at the group at the start of a session. I don't think that's okay. It may have been well intentioned, but its still a wrong thing to do.
Sorry (not sorry), but you don't get to insert your own npc's into my campaign without asking me first.
I mean, they guy didn't just sit at the table and start telling you who these NPCs were right?
That is exactly what he did.
He probably handed you a document of names and background sketches before or after the game?
He tossed me a handful of paper at the start of a session, with extensive background stories on all the characters as well.
Maybe an e-mail sent to the whole group? This is the type of phrasing that makes it feel like you are upset, that he somehow crossed a line.
He did not send an email.
And, by horrible, I was worried that he had written characters along the lines of "a secret runaway princess deeply in love with my character." That sort of thing I can see upsetting people. But if they are perfectly fine characters, just not your characters, then what he did doesn't seem that bad.
I don't feel like the quality of his writing is the concern here. Its the act of taking over some of the DM's responsibilities, without permission from the DM or the rest of the group.
See, this is the sort of detail that clicks things into place. This isn't just a player making NPCs, this is a former DM pushing his NPCs into a new game. A game partially or fully made up of people who quit his last campaign in part because of how he pushed his NPCs, giving a repeated pattern.
They didn't quit his campaign until a few months ago. This debacles happened way before they quit his campaign. Although, an argument could be made that there was already some dissatisfaction about his campaign hanging in the air. If this motivated my players to be upset with him, I do not know for certain.
Heck, if you'd led with that I wouldn't have even bothered with most of my response, because this isn't a player getting inspired, this is another DM seeming like he is trying to muscle into your turf. Of course this is going to raise hackles, especially with a group of players who quit his game.
But I don't feel that was the reason my players were upset. They were upset, as I understand it, because they were playing my campaign, and he took it upon himself to write characters for me, without my permission. And not just a few characters, but a lot.
Sidenote: It is a lot of work to edit out so many of your replies to other posters when you put them all in one post. Especially when I'm on my phone. Could you separate your responses into individual posts?