What if you don't use Galaxy? I think I'll hang on updating my games until I know the offline installers work OK.
So I downloaded all the new installers yesterday. I tried some of them out, and when I try to execute the offline installer for Wing Commander (1), Windows tells me that the file is defective and recommends me to get another copy. Which didn't help. I tried redownloading the file manually and via Galaxy's backup option. Same result. Installing the game via Galaxy worked, though.
After that, I tested some of the other installers (Wing Commander II, Wing Commander: Privateer and Ultima VI), which worked just fine.
One thing I noticed: when you select the "launch" option once the installation process is complete, Galaxy will be launched if it wasn't active. If you simply close down the installer and then launch the game via the desktop shortcut, Galaxy will NOT be launched.
EDIT: This issue has been fixed!