Yep, you pretty much nailed that!
It can be a frustrating tool if you don't understand it, that is for sure. A couple of comments/clarification.
Media Center temporarily imports files as per the selections at the top of the MC Explorer. It is that part of the database that MC is looking at when you view the directory structure in MC Explorer. MC shows the files just as they would be if they had been imported, using the same rules such as reading data from file names using CARNAC, and reading tags inside the files, as you mention. It is not a file explorer like Windows Explorer. What is shown will be the tags/fields as best MC could determine when it imported them into its temporary database.
I'm surprised your thumb drive didn't show up in MC Explorer pretty quickly. I guess you had MC Explorer selected when you inserted it. All you needed to do was refresh the view and it would have shown up. You didn't need to play the files. MC does actually rely on File Events to identify if it needs to recheck a drive. That process is very quick. If File Events aren't supported then a refresh, or some other way to trigger MC to update, is required.
There are some other "GotChas", such as.
Using the Explorer on a MC Client while it is connected to a MC Server can be very confusing, as MC looks at the local hard drive names, but shows the remote files under drives with the same name. For example, I have an F: drive on my Client. My MC Server has an F; drive as well. There is no F:\Movies sub-directory on my local drive, but when I select the F: drive in MC Explorer, and then tick "Show Files In Subfolders", MC displays the files on my MC Server. That is logical, because MC is just querying the Library using the [Filename] field, and not the hard drive. The Library shows files in F:, so MC displays them.
Put simply, using the MC Explorer while connected to a MC Server Library can be
very confusing, and makes little sense. However using it when a local Library is loaded can be very helpful, if used carefully.
It is a great tool for checking what is on your drive, and of those, what has been imported into MC and what has not.
It is an excellent tool for tagging files before they are imported into MC, so that the original import has the correct tags. That could be used to fix the "Track 1" name situation, for example, before MC ever actually sees the files via a real import process.
It can be used to fix tags inside files which will never be imported. MC just writes the tags as if they were imported into its normal Library, and then forgets the files when you close MC Explorer. Neat.