You are looking too hard to find compromise by insisting that the only people who have consistently talked about working with players when possible to find some workable compromise.A player wants to play a tortle . . . simply because they like the idea of playing a turtle-person, is a very reasonable reason. IMO. It doesn't need to be any more involved that, "That sounds fun!" Good enough!
A DM who doesn't want tortles in their game . . . maybe they do have a good reason! That's what Session Zero is for, to hash out that kind of stuff. But IME, a lot of DMs ban things like tortles arbitrarily and without good reason. IMO, of course.
Now, is that the DM's call? Yes. Is it my call to roll my eyes and walk away? Yes.
Are the two preferences equal? Player wants to play a tortle, DM doesn't like tortles? IMO, no. DMs set the stage for the game . . . but the players should have primary agency over their characters.
If you shoot down my PC concept and your reasoning (IMO) sucks . . . you are putting your "setting consistency" over my player agency to create and control my own character. If you have a good reason and clearly communicate it to me . . . I'll roll with it.
Of course, we also have to consider the other players at the table. If you are the one player who is all about turtle people, and the other players would rather play a grim-and-gritty urban thieves guild adventure . . . but again, that's what Session Zero is for. If there is a disconnect, someone has to compromise or decide to walk away. Either choice is fine.
That collapses when faced with the insistence that no compromise is possible because it's not called a tortle with the body structure statblock and culture of tortle. The reasons you quoted from mamba are the capitulation or bust ones that prevent any compromise not a starting point for rejection.
Heck at one point someone even insisted the gm needs to allow it because it's difficult for a player to bring themselves to say they don't want to join a game


