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The door number thing is no more "totally incorrect" than what you posted about DR is totally correct. It depends on... many things, and what you say is imprecise.
Generally speaking, if you're DR B or above, your door number is your rank in the lobby. It doesn't work with maxed out A+ drivers, or lobbies with mixed DR D/C drivers because it's sorted in what's known as reverse alphabetical sorting; all numbers with the highest first digit (9) are ranked above all numbers with the second highest first digit (8), and so on down to the lowest first digit (1). That means a DR D driver with a rating of 9 will have a lower door number than a DR A+ driver with a rating of 100,000; although this is incredibly unlikely to happen, it shows why it works perfectly in a range of 10,000-99,999, and is all over the place in a range of 1-9,999.
It shouldn't be taken as an indication of "where you're expected to finish", but where all drivers are DR B or above it very much is your ranking and suggests who you need to beat: people with lower door numbers than you.
Your ranking is important because it clues you into the DR score of other drivers, and that drives the points scoring, along with finishing position. In essence you gain some points from everyone you beat and you lose some to everyone who beats you. That broadly means that if you finish top half you gain from more people than you lose to but it depends on the difference between your DR and the other driver.
For every driver you finish ahead of, you gain 80 DR plus ((their DR - your DR)/500).
For every driver you finish behind, you lose 80 DR plus ((your DR - their DR)/500).
Now that might mean that if you have a DR of 40,000 and beat 14 drivers whose DR is 1 you might gain almost nothing from them, and if the one guy ahead of you also have a DR of 1 you'd lose 160 points to him. That's a net DR loss of 146 points from finishing second. On the flip side, if your DR is 1 and everyone else's is 40,000, you'd gain 146 points for finishing second last.
It's also not cast-iron fact that you gain/lose more in FIA or Race C; you might, simply because there's more drivers to finish ahead of (or behind), but there's no points multiplier for these races. You still gain and lose at the same rate.
Ultimately what you need to do to gain DR is beat as many people as you can who are higher rated than you, while losing to as few people as you can who are lower rated than you. Losing to people who are higher rated doesn't have as much effect, nor does beating people who are lower rated. And, so long as your DR and that of everyone else in the lobby is above 10k, the door number clues you into who those people are.
Finishing top half is definitely a good place to start, but absolutely not a guarantee.
