Nope, can't downgrade a Kindle

.
See the relevant sticky/KUAL FAQs for how to get stuff back into working order (basically: hotfix -> KUAL coplate -> update all the things).
On a jb device, there *might* be convoluted ways to bypass the downgrade restrictions, but they're untested, unsupported, and potentially unsafe on a non-empty device.
Even then, 5.12.x might have nixed those, too, anyway.