Experiment with some files you don't care about.
The Shift Delete key pair is different than Delete. Read carefully. Go slowly.
I did the test then I did the whole thing now the drive that the files are in is not even showing up. The files are still there. JRiver said after the update it had 24932 files in the library but I sure as heck can't find them in the library.
Here are some screen shots. Fist one shows lots of files in the main library.
This one shows zero files.
This one shows how the import is set up but still no files showing.