But why does it end up washing out colors unless I amplify them in kwin? Is just the brightness absolute in nits, but not the color?

The desktop runs in SDR and the color space differs between SDR and HDR, meaning you will end up with washed out colors when you display SDR on HDR as is.

When you increase the slider in KDE, you change the tone mapping but no tone mapping is perfect so you might want to leave it at the default 0% and use the HDR mode only for HDR content. In KDE for example, colors are blown out when you put the color intensity to 100%.

Why does my screen block the brightness control in HDR mode but not contrast? And why does the contrast increase the brightness of highlights, instead of just split midtones towards brighter and darker shades?

In SDR, your display is not sent an absolute value. Meaning you can pick what 100% is, which is your usual brightness slider.

In HDR, your display is sent absolute values. If the content you're displaying requests a pixel with 1000 nits your display should display exactly 1000 nits if it can.

Not sure about the contrast slider, I never really use it.

Why is truehdr400 supposed to be better in dark rooms than peak1000 mode?

Because 1000 nits is absurdly bright, almost painful to watch in the dark. I still usually use the 1000 mode and turn on a light in the room to compensate.

Why is my average emission capped at 270nits, that seems ridiculously low even for normal SDR screens as comparison.

Display technology limitations. OLED screens can only display the full brightness over a certain area (e.g. 10% for 400 nits and 1% for 1000 nits) before having to dim the screen. That makes the HDR mode mostly unuseable for desktop usage since your screen will dim/brighten when moving large white or black areas around the screen.

OLED screens simply can't deliver the brightness of other display technologies but their benefits easily make it worth it.