In Synaptic, when I click
Add downloaded packages and navigate to the folder with the debian packages, the debian packages are greyed out, which means I cannot select them.
According to this question you are misusing this feature. Apparently, this feature in Synaptic is not for installing individual DEB package files.
To do this - without using the command line - navigate to the folder where the packages are and double-click one (or more?) of them. This will bring up the Package Installer and you should be good to go.