I'm puzzled by several things in your post.
you've elected to save video tags to sidecar files, even though you're also using PvdImport for video meta data.
Why should the use of PVDImport and the use of sidecar files have anything to do with each other?
the possibility that the incorrect sidecar file was somehow written to the new file's directory
All my movie files are in the same directory. Hence all the .xml sidecar files are also.
or are you using MyMovies to create sidecar files?
No, the sidecar files are being created by MC. The two new movies both have .xml sidecar files with time stamps at the time they were imported. And both contain the wrong information. However, I did try MyMovies at one time in the past, and when I looked, I found a file called "Parent Trap Lindsay Lohan.xvid.mymovies.xml", which looks like it came from MyMovies, and this is the movie whose information was used incorrectly in the new movie's tags. There is also a file "Parent Trap Lindsay Lohan.xvid_avi_JRSidecar.xml" from MC. Are you suggesting that the presence of this MyMovies .xml file is screwing me up? I can easily delete it if that's the case.
Have you investigated those sidecar files as the most likely source of the erroneous tag information?
Well, yes, it seems likely. But why would the new movie be tagged with information from a sidecar (or other) .xml file from a different movie?
And none of this explains why the file "s18e03p1-4 Full Circle.avi" was imported with the name of "Doctor Who - Full Circle - Part 1" while the file "s18e02p1-4 Meglos.avi" was imported with the name of "s18e02p1-4 Meglos". And the file "s16e03p1-4 The Stones of Blood.avi" ended up with the name "Dr. Who - The Stones of Blood - Part 1"