Mac – Which websites provide alternative listing of Mac App Store content

At present the app isn't set up to restrict its spotlight search to any specific volume. That behavior falls as a natural consequence of "the Mac way" of letting an app reside anywhere and still run.

You can use disk utility to unmount that volume when it's time to install or update as a workaround.

Typically, the method that you described will work. However, in some (rare) cases, the app will change the way it stores settings and other information, and that would cause the old version to misbehave until you delete the app's settings (usually found in the Library folder) and start anew.

But, as far as the App Store goes, your method is pretty much the only way to go, and the App Store shouldn't complain if you reinstall the old version.