If I install lubuntu-desktop to try Lubuntu and then later remove it, will my normal Ubuntu be safe, unchanged and uncluttered?
It is indeed possible to fully retain your Ubuntu desktop after removing it. But when you remove (or purge) the lubuntu-desktop after installing, chances are, the applications installed by Lubuntu will still be left behind.
Check the following question on which applications to remove to get rid of Lubuntu apps:
Also, go through the entire command because it might remove applications which you might wish to keep.