1, 2. Not a bug, in that no extra options will be imported. It's just leftover stuff hanging out. We did make a change this year to remove the unnecessary stuff, since it wasn't doing anything.
3. We throw up that dialog mostly as a precaution, since export/import is not the ideal way to migrate a configuration from version to version. There were no changes in the formatting of the export/import of the Extensionless File Manager between v18 and v19, so you should be fine, and as I said, there aren't actually extra options being imported.
4. Would the Extensionless File Manager be useful to you on a per-project or per-workspace basis, rather than doing a import/export on an as-needed basis?
5. I'll check on this.
Thanks for all the feedback!