if I install bblean or litestep, will I be able to get the default shell back by uninstalling the custom shell?
I've personally used both of these applications; as long as they are properly uninstalled (by using the uninstaller app that comes along with them), they'll revert you to the standard Explorer shell afterwards.
If you're really worried that something might break, take a system restore point before installing.
You can also manually set it in the registry, at
I don't know these products but unless they delete some of the Windows system files (which I doubt) you can always switch back to the Windows Explorer shell.
There is a single value in the registry which determines which shell to start, you don't even have to uninstall the alternative desktops.