I would use this method to set the default kernel prior to running the upgrader to upgrade to 17:
Before booting into 17, I would then boot into the Recovery menu and ensure that the Grub File is set to as you set it above, not reverted to default.
I would then ensure that the 5.15 kernel was installed.
I would move back up to the boot menu and ensure I was booting up on Zorin on 5.15.
Upon reaching the GDM Login screen, then ensure you are logging in on Zorin on X
, not on Wayland.