Strangely, I enabled it a year ago and jpegxl.info is still displaying that my navigator does not support jpegxl
Only in nightly unfortunately.
I don't think so. However, you can write a magisk module that inject content into /system.
Have you tried Shizuku or magisk's libsu ?
They both allow you to run services as root (or adb shell user with suzuku).
Strangely, I enabled it a year ago and jpegxl.info is still displaying that my navigator does not support jpegxl