Hmm... sorry, but what is your problem now?
You have imx_usb_loader working, which can boot your kindle into any mode that you want.
You also got fastboot working.
In other words: you have complete control over your Kindle, whichever state it is in.
If I understand your question correctly, you only have to boot into diags mode (using imx_usb_loader), and to follow the other directions in the debrick guide.