[NiLuJe]

Yep, like geekmaster said, you should be able to do a reset to factory defaults through the recovery, which should help, unless you broke some boot scripts.

I'll try to find my few posts about the procedure on a K3, but basically, hold [HOME] after a hard reset, and type RESET when the reset screen asks you to.

EDIT: Err, that might be the procedure on FW 2.5, not 3.x, can't remember... >_<"

On the recovery update front, forget about it, they need to be signed by amazon, and in the right format. So, while kindletool can build recovery updates, you won't be able to actually run them (because they'll be signed by us, not amazon), unless you have a patched kernel.
No idea what happens with OTA official updates, but, if it even accepts them, I doubt the result would be different than if you had run it through the Settings page (it won't help in your case, but it probably won't hurt either). And, as geekmaster said, I've never seen official recovery updates in the wild. (And there aren't much custom ones floating around here either, since it implies running a patched kernel).

EDIT²: If I remember correctly, the thing with FW 2.5 (and possibly the K3), was that to enter the recovery menu, you *had* to go through the RESET screen... (And then hold [HOME] again to get into recovery, I think).

Anyway, if you've already done the RESET=>Recovery thing, and it still doesn't boot, you borked a boot script/a vital config file. If you had usbnetwork installed, you might try to put it back on the userstore (the usbnet folder, not the .bin) and enable the auto mode, then quickly ninja ssh on the next reboot and stop the framework before it triggers the 'YKNR' screen, and then see what exactly is the problem. (You mentioned 'tweaking' some boot scripts to enable usbnet at boot.. Err, WTH? The feature's builtin the usbnet hack, you just have to touch a file in the userstore... o_O)

Other than that, serial console ^^?