Displays are indexed 1, 2, 3... left-to-right -- but you can also use -1, -2, -3... to go right-to-left. So
%ScreenVisible%-1% will always get you the bounds of the rightmost screen, however many are connected.
The Applications Palette is a Keyboard Maestro engine, but hiding/showing it doesn't gives us a "Last Engine Window ID" we can use But we can get it with AppleScript -- it's the Engine window named "Applications" -- then use the ID in a "Move Window" action.
Here's a demo:
Move App Palette.kmmacros (2.9 KB)
I only have one screen, so it's all rather theoretical -- but it does work on a single screen!
Note that if you ever pop up a "Custom HTML Prompt" window or similar titled "Applications" there's a chance of confusion, although the App Palette should always be the first item in the list. It's a pretty slim chance so I haven't included any error checking, but bear it in mind if the macro misbehaves...