Hi!

Is there any possibility to extract a slave site for use it as standalone?

YES.
Follow the backup/restore procedure as described in the FAQ.
www.jms2win.com/en/faq/faq-joomla-multi-sites#cat-268

The tips consists in doing the backup with a tar -chzf where "h" means replace the symbolic links by a physical path.
So that, when you will restore the slave site, you will have the PHP code.
Once the backup is restored, you can uninstall JMS that might be present in the slave site and also remove the JMS patches that are present. Becarefull that when you uninstall JMS, this restore the backup of some files (including the configuration.php file that is the one of the master). So keep you slave "configuration.php" to be able restore it after the uninstall of JMS.
To remove the patches, we also recommend that you redo an unzip of the standard Joomla package over the restore site. This will remove all the JMS patches installed in the "core".

Concerning, the DB and the "configuration.php" of the slave, JMS does NOT touch them except if you have decided to share the content of some extensions.
In this case, the DB may content sharing of tables instead of the physical tables.
If this is the case, you will have to export with PHPMyAdmin the physical tables and restore them into you slave DB after that you have changed the table prefix of the physical table to restore.
If the slave DB didn't had any sharing, you don't have to do anything on the DB.

Next question: How do you remove the site from the JMS component? Is it as simple as viewing the slave sites in Manage Sites, selecting its radio button and clicking "Delete"?

I just extracted a site as delineated in your backup/restore procedure and it worked, but it brought in all of the images from the master site's images directory.

Now I'm wondering how to remove the site from JMS Multisites?

What about extensions which only the extracted site used? Can I just uninstall the extension in the master Extension manager?

The JMS instructions mention the issue of "ghost tables".

By the way, a problem I ran into was with not having set Media Manager to use the images_specific directory as the upload root in the slave site. Which meant that I had to search the slave's database for images/ and find image files in the master images and move them, then correct the image path in the database.

I don't recall instruction in the documentation advising of this important setting.

Yes you can just delete in the JMS manage site.
Just becarefull when you select to also delete the DB when this DB is still used by the standalone website that you created.