@variousvirgil: Why do people keep trying to install both the bridge and the JB? (That's not a rhetorical question, I'm quite keen to know what in the docs leads you to believe that's needed).
That said, yes, you can downgrade from diags, just get the kernel & rootfs from an official update w/ kindletool, and flash it manually.
This thread details the procedure, albeit for a different device/FW version, but that's still relevant, just use the proper files.
Make *absolutely* sure you're using a FW version targeting your device, or you will shoot yourself in the foot.