I think you nailed it here; D&D relies on a ton of cooperation between DM and players (and between players and player) to function and be really enjoyable, so the social contract really is Rule 0.
True. It also comes with the reality that 100% of the players are homo sapiens. Then it goes downhill from there because tabletop gamers include a disproportionate representation of homo sapiens with enhanced lack of social skills.
If you can't be bringing the cooperation to the table, even when you think someone else is the problem, then you might want to consider a hobby outside of tabletop RPGs.
In the strawman case presented, the DM is being a bit of a dick. But, frankly, Player A is as well. Every example that Player A offered was some form of special snowflake that was going to have impacts throughout the game simply by its presence. Now, it is certainly ok, cool even, to want to roleplay that kind of super-special character. But it brings baggage with it. And if a player absolutely insists on it then THEY are bringing baggage. It doesn't leave much room for arguing about other people's "social contract" obligations.
At the end of the day, the DM is either good enough or not good enough. If there is an awesome DM with a slot open, but I am required to play a kobold rogue. I'm there.
If I can play anything I want, but the DM sucks, then it doesn't really matter what I can play because I'm going to do something else.
Reality will be somewhere in between. But to take a requirement of being able to force any option into the DM's world or campaign as a litmus test is just stupid.
If the DM says you can't do this, don't complain. If you don't know any better then give the DM the benefit of the doubt. If you know the DM puts on a good game, then get over it and start having fun. If you know the DM sucks, don't complain, just be glad for the early reminder and move on to something that is fun. If the DM has a reason, the players should respect that because the game IS a social contract and reducing the fun for everyone is not a fair trade for you not having to settle for not being the special snowflake. If the DM doesn't have a good reason, then basically the DM is being a dick. In this case, it won't stop there and it is safe to assume that this attribute will manifest in other parts of the game. So you are back to be happy that you got the early warning.
But it goes in reverse. If I am at a table (as a player or DM) and one player is being demanding like Player A, and won't be agreeable, then I am GLAD when they walk away. I'd rather lose them than have them bringing everyone down. This applies even if the DM is just a jerk. The game may be doomed anyway, but adding a player that won't work with the social contract doesn't solve anything.