Thanks, ixtab, you're right, "idme -d --bootmode main;reboot" made no difference.
So that overwrite on-device boot loader is the only choice if I want to save my half-bricked KT. But the thing is I am totally unfamiliar with linux, all I know is simple command like ls, cd, etc.
That link leads me to some source code files, I downloaded the Kindle_src_5.0.0_1370280073.tar.gz for KT, and there is a uboot_2009.08.tar.gz file in the zip file, but it's not a .img file. For what I know if boot into fastboot mode, the bootloader is a partition like kernel and diags, don't I need to input command like "fastboot.exe flash bootloader uboot.img" ? or just input "fashboot.exe flash bootloader uboot_2009.08.tar.gz"?
The u-boot wiki lists some linux commands for reference, I'm not sure if I can deal with it when it needs to do something more or less. Appreciate if I've been given more instructions on that
