Cycling two abreast is better for the driver, since they can overtake much quicker.

I disagree since overtaking a cyclist in the same lane is unsafe anyway. In the city I always cycle in the middle of the lane because it prevents unsafe takeovers and dooring.

So it's a bit of a conundrum. Because there are pros and cons in riding abreast.

On one hand, cyclists are more compact and more visible. On the other filling whole lane would mean drivers behind them would have to time their overtaking. However, car drivers almost never leave enough space when overtaking cyclists and 100% never think about wind that might push them or that cyclist might need more space to avoid potholes and stuff. So being a dick driver is not exclusive to cyclists.

Traffic law, at least where I live, states when overtaking cyclists driver must leave enough space between him and the cyclist so as to not inconvenience cyclist. Which is vague and not helping one bit. However I think it's far better to be forced to slow down and time overtaking than not slowing down and flying next to a single lane of cyclists. Because if and when there's a car coming from opposite direction, car driver won't care or look twice to move closer to the edge of the road and push others out.

Yes illegal, depending on the country.

Where do you see another vehicle "waiting to pass"? There's absolutely nothing in this picture telling you how much traffic there is, how wide the road is, etc. Nothing.

What can be seen in the picture, however, is a car that, no matter the speed, is tailgating way too close. Which is a misdemeanor in some countries.

Mostly one country.

Fun fact, humans actually produce only trace amounts on methane in our farts because we're not ruminants. Most of our farts are nitrogen swallowed from the air and CO2 produced by gut bacteria. The bad smelling chemicals are in even lower concentrations and barely make up a rounding error by volume, we simply evolved to be really sensitive to them because it's beneficial to our survival to avoid poop.

You are assuming drivers respect the safety distance from a lone biker...

It does matter. It's safer for everyone if cyclists travel side by side in one lane because then the car driver has to spend less time in the oncoming lane to complete the overtake. A long string of bikes takes more time to safely pass.

Using millimeters is pretty common in engineering.

Yeah, that's the most unrealistic part of that. Almost every car here has only one person in it.