I received a message saying that the package doesn't have a stored checksum value when attempting to install it. Is this normal?
Any archive created through the package manager may it be backups or installable packages, contains checksum stored inside the archive itself. Whenever you install a package from an archive or restore a backup, the package manager performs a checksum matching of the one currently stored in the file and the checksum of the archive itself and if it fails to match, the program will automatically abort the operation.
There’ll be cases where an archive doesn’t have any stored checksum. Normally, this is caused by archives created through 3rd party programs. When you attempt to install a package that doesn’t have a stored checksum, the program will warn you about this and will ask you if you want to proceed with the operation.
TL;DR: Yes, it is normal.
I want to reinstall a certain package but the reinstall button is greyed out. Why is this happening?
This happens when the archived reinstaller files of the selected package is missing.
I uninstalled a package and now a system image/character body sprite is missing. How do I fix this and is it a bug?
This is not a bug and this happens whenever you uninstall a package that replaces these files. The package manager will not automatically replace the deleted system image/character body sprite with the default one.
The only way to fix this is by installing/reinstalling a package that has a custom system image/character body sprite or by reinstalling the default assets package.