after some experiments, I guess my Kindle (4, nontouch) is broken pretty good. Fun is: I didn't even try anything strange - it froze while trying to connect to a network and then never bounced back.
Originally, i could reboot it to the startscreen, with the loading bar stuck at about half. After some time, it rebooted. It did that again and again and din't care whether it was connected to PC, Power supply or not at all.
After finding this forum, I was able to use MfgTool to boot into debug mode. Unfortunately, it froze soon after. With some more tries and quick copying I was able to activate USB-storage and to get the "ENABLE_DIAGS" file onto it. This way, I at least could try some stuff before the kindle froze. Still, it didn't help much.
Starting fastboot and connecting via fastboot worked (I installed the driver via inf-wizard in libusb). That way, I at least knew that some parts still work. The kindle doesn't seem to crash in fastboot but - of course - it's not of much use in fastboot. I can verify the information from getvar (mac, serial, product) and the kindle responds to information from here but it doesn't help me getting it up and running, again.
"fastboot check" reports CRC-failures on bist, userdata, kernel, diags_kernel, system, diags and data. Even after doing a "fastboot erase diags", check fails. Same response after "fastboot flash diags mmc..." (I dd'ed the partitions from another kindle 4nt as I didn't find a diags backup online) - although it sometimes fixes that "not booting to diags but to black screen" problem.
I tried flashing "system" with mmcblk0p1.bin and although fastboot pretends that is succeeded (still check fails), I've read that fastboot 350MB is too much for flashing via fastboot and should be written via dd - once you can use usb-net (which I can't because diags mode freezes before I can do anything).
For the record: I tried bootmodes factory and reset and although my kindle did "something" after reboot, it refused to load correctly.
Some little thing that confused me: The "Adjust battery" in diags mode often reports a negative battery charge current. Still, that screen assured me that charging the battery works. When the Kindle is connected to a charging device, the device battery level rises.. (up to 100 where it is at the moment).
Very many thanks for all the documentation and tools that have been compiled in this forum. Without these, I'd have given up some time ago and I guess this will prove useful if I ever decide to toy with my next Kindle.
I'm pretty much labeling this Kindle "broken beyond repair", so if anyone has some ideas, experimental kernels or stuff like this, I'd be more than happy to try it.