First of all you should know you can run "add to inventory" operation just for .vmx (VM configuration file) not other types like vmdk.

BTW I think you complicate it, because it's easier to create a clone and do whatever you need, then remove that clone or create another one for another test.

Snapshot is not simple as you may think. This operation include some interaction and modification about .vmdk, .vmsn and .vmsd files. If you like to know read my post here:

Virtual Machine Snapshot Details Investigation - Part 1