I think that's where the "compromise" framing falls apart. I am look for a compromise of "How can I play the character I want in a way that won't upset your world-building", which I have given several options from mutant to magical creation to super-rare species previously unknown to "came off a spaceship that crashed." You have responded with compromises that don't involve me actually playing a tortle, but just a regular dude with a turtle fetish. That is like saying "You can't be a wizard. You can be a fighter with a stick and robes who hits people with the stick and yells "MAGIC MISSILE" when he does so." My compromise is "how can I make this work in a way that doesn't break your world" and yours is "how do I make it so that your character is nothing more than a funny hat on my already in stone preferences."
It ends where the player and DM find actual compromise. DMing 101 teaches two improv skills: "Yes, and" and "No, but". Yes, And would be the "Yes you are tortle, and you are a one-of-your-kind magical experiment". No, but is "No, but you can be a lizardfolk who developed a shell-like curse and everyone thinks he looks like a tortle." In both scenarios, the player gets his character and the DM is consistent with his world.
But that's not what is happening. We are stuck with just "No." Or maybe "No, but you can put on turtle Spirit Halloween costume and people will look at you funny."
Depends. Can I refluff the firearms are modified crossbows?
Clearly the GM is referee for in game actions, but usually a GM listens to both sides before ruling. Then again, I would wager if you weren't willing to listen to me on character generation, I feel I have little chance of winning any disagreement about rulings.
Who are these mythical anti-tortle players whose whole night is ruined if they see a tortle? Do they sit there and police every other PC as well? "Sorry Bob, I won't play with a warlock. Make a new character." "No Sue, you either ditch the paladin or I walk!" "I can't stand halflings, too Tolkien. If anyone plays a halfling, I'm killing them on sight!"