Use the same UI but remove the Install button
Just mix the installed nodes with the available ones (imagine the two images above are combined into one)
The main issue (for me) is not remembering to click on Install option - so I type simplet and simpletime doesn't show up as an option as its not installed - then I remember to click Install but I have to re-type simplet
If just one common list - then no confusion for befuddled muggles like me
The entries could prob stay exactly as they are - maybe add an explicit installed label on the installed ones would make sure no-one was confused?
Maybe some mileage in putting the installed ones at the top??