None of these really have anything to do with backstories
Gm: this is going to be a grim dark gritty raven loft campaign.
Pc1 can I play a turtle?
Gm:yes but see grim dark raven loft
Pc1 shows up with ten pages of nonsense about how his tortle donataro is on a quest to avenge his family name after his father high king eliminated of water deep was disgraced by scheming backstabbing business partners.. oh and he wears a purple mask to match the purple grip on his quarterstaff. At every opportunity pc1 will remind someone "well on a role player and my character would.... " or try to call upon high king eliminster's name then argue with NPCs about it no matter how often the gm reminds him grim dark ravenloft so they don't exist and he was told.
Is just playing Donatello and trying to play the wrong setting.
So, is either trying to be disruptive or actually has no idea what setting Ravenloft is and got confused. Either way, nothing to do with backstories.
And actually, considering Strahd constantly pulls in people, a background where Donatello has been seperated from his real quest and has to finish this so he can escape and continue avenging his father could work really well. If the player is taking things seriously.
Pc2 says "I'm an Everton style orc bard with cadet background. He shows up and dives into the grim dark ravenloft campaign where he discovers his character along with everyone else to fit the campaign as it progresses. . All is great.
So no backstory is the best backstory? Except, there has to be something there since he is an Everton style Orc and a Cadet, which implies a military background.
So, there actually is a backstory here, just not one we are getting told. So again, not a great example of anything except that the DM seems to have no problem with this player.
Pc3 is also an Eberron type orc cadet bard but has four pages of msrysue fanfic about how he was wronged as a cadet and framed by corrupt superiors flto cover up their embezzling. He too says many of the same things as pc1 and never makes any attempt to join the grim dark ravenloft campaign or tie his pre written backstory to either the campaign or any other player no matter how many olive branches are extended to the party's second broken latch. Like pc1 he expects an 18 level ravenloft campaign about unethical human experimentation and the ramifications based loosely on an old racenloft module/supplement about that to significantly involve his corrupt boot camp instructors from $notbleepingravenloft
So, again, this has nothing to do with Backstories. The main complaint is that the player seems to not want to play Ravenloft. OR again, doesn't know what Ravenloft is. On top of spotlight hogging and probably a whole host of other things.
And if the DM told them it was going to be about unethical human experimentation... dang if PC1 and PC3 missed some hellaciously rich backstory opportunities. The Turtles were originally about pollution mutating animals and people, so you could easily have Donatello as an escaped experiment. The Corrupt Military being involved in the experiments, PC3 finding out and that being why his corrupt superiors booted him. Could have been great, and really tied things deeper together.
See, you can't blame backstories for players who aren't playing the game. Because if they were going to be disruptive anyways, then the backstory makes no difference.
In fact, PC2 could just have easily walked in with no background, and did what a group I played in did during a Ravenloft game. Kill anyone who inconvenienced him. Quite literally, a woman locked her door and was terrified to let us in. They broke the door and when she attacked them, they killed her in "self-defense"
Then did it again after my cleric used raise dead to bring her back.
But they didn't write backstories for their characters.