I've been lucky to jailbreak and get KUAL working on Kindle Paperwhite 3 (G090G1) @ 5.9.2.0.1 firmware. I've got my PW3 on this firmware version initially. Downgrade to 5.7.4, jailbreak, reupdate and KUAL/MRPI as well as extensions setups succeeded for me.
The steps I followed are on below 3 links.
Nuance is that after downgrade-jaibreak-reupdate, following steps of 3rd link KUAL did not appear in the Kindle's Homescreen. After reading this thread I was about to leave this thing for better future. But...
What I did is, I factory-reset my device, repeated jailbreak procedure described in 2nd link and attempted to follow 3rd link using "For 8th generation Kindle .... " 7-step-way described in section 1.1 instead. And it worked - KUAL appeared in my Kindle's Homescreen, I was able to install other extensions following further steps.
1. Downgrade to 5.7.4:
https://www.ereader-palace.com/downg...n-2016-method/
2. Jailbreak procedure
https://www.ereader-palace.com/how-t...e-2016-method/
3. KUAL/MRPI
https://www.ereader-palace.com/4-mus...eaking-kindle/