1) The ID3 tag reader has not changed significantly in MC16. If you create a new library in MC15 you should see the same behavior as with MC16. To understand why the tags are not read as expected more information is needed. In general, ID3v2 tags have a preference over ID3v1 if both versions exist in the files. By default MC writes both and keeps them synced, except that ID3v1 can store only a few tags and has serious limitations (
e.g. maximum length, restricted genres, no UTF character encoding support).
2) The "Install Drive" location is used with the Portable Install option. Did you actually select the Portable option in the MC16 installer? A Portable installation has some limitations, though "files do not play" is not supposed to be one of them. Once again, more information would be needed to understand your problem.
3) This would indicate that the files were not detected as MP3 audio files during the import. That is very odd. What happens if you simply drag and drop a couple of new (unimported) MP3 files to the Playing Now list? You can create copies of some existing files to a new location to test this. Can you play them?