Several of your musts and wants seem to be about ancient features or limits of old Kindles. Many of them are available or no longer an issue on slightly newer models than the K3 and K4. Is there a particular reason you seem to be focused on those? You can get a Kindle that runs firmware 5.9.6, which adds user font support, or 5.13.5 that adds book cover support as long as the Kindle is not ad-supported.
Margins was a problem of the ancient and very limited MOBI format. You should be able to use AZW3 or KFX and get the margins aligned as you want. Even better, you can jailbreak all reasonably modern Kindles (for now) and install KOReader and have a highly configurable reading program that will read non-DRM EPUB and PDF directly.
Only the early models with a frontlight couldn't be turned off. I don't recall when they stopped that, but it wasn't too long after. I want to say the PW1 and PW2 were the only ones but that's just a guess.
You can reference which models support which max firmware versions
here.