Hi, Phung.
I'm only an user like yourself, but here's something you might try (since KUAL seems to be working for you).
Go to this thread
https://www.mobileread.com/forums/sh...d.php?t=203326 and get the "Helper" extension. Once you place the "helper" folder inside the "extensions" one inside your Kindle, disconnect it and enter KUAL. There should be a "Helper" tab there, click that and then click "PREVENT ScreenSaver". That should do the trick (but don't forget to click the "ALLOW..." one once you're finished your reading).
About the files meant for KOReader only opening through KUAL, have you installed KPVBooklet? Maybe you should try to reinstall it if you already did. Here's a link:
https://github.com/koreader/kpvbooklet/releases
This one should be installed with KUAL or the ;log mrpi command, by the way, since it's not a KUAL extension.