Yes pamac can be called with or without sudo but the recommended way is indeed without.
Anyway that should not make a difference except for building packages from AUR because makepkg cannot be run as root.
Warning Never use sudo with pamac. It will ask for escalated rights if it needs them
Updating the kernel doesn’t mean replacing an unmaintained removed kernel (like all the non LTS kernel that live a few months in Manjaro repositories).
Updating the kernel means that the kernel you are on (let say 5.15 for example) will receive its updates.
Pamac will not replace your kernels the day they are removed from Manjaro repositories. It is on you to follow the update announcement threads to keep informed of the supported kernels (basically if you have a LTS kernel it will live many years, while other non LTS kernel like the “latest and greatest” stay a few months until they become EOL).
Yes, you need to choose the kernel version you use. You probably won’t need this in a point-release distro like Fedora because they usually stick with single stable kernel version for each stable distro release, but choosing kernel version easily is something you’ll find in rolling-release distro.
You can install multiple kernel version in Manjaro by mhwd-kernel or from the GUI settings.
Pamac only updates kernel security and bug fixes, not the kernel version.