Hey, thanks for posting your concerns about Spotify's system of matching local files to the Spotify catalog. I will try to answer your questions as best I can.
Yes, Spotify should attempt to match all the tracks you've added locally, meaning when you add one of these tracks to a playlist on one device it will show up on other devices if it has been successfully matched. If you then play that track on another device through normal streaming, it will play Spotify's copy of the song, not the copy from the original device you added it on. That said, if you have Spotify open on both devices and are also connected to the same WiFi network on both devices, if you download the playlist on the second device for offline listening, it should sync the local tracks that are on the first device, including any tracks that don't happen to be available in the Spotify catalog. Please check out this page for details about working with local files, particularly the section on syncing them to a mobile device or tablet.
There is definitely a difference between free and premium, as you would need premium to actually have access to the download option; in other words, if you're on a free account, your other devices would only have access to the versions of the songs that have been matched and are being played from Spotify's catalog.
If the track has been successfully matched, yes, you should still be able to listen to it as long as you've saved it to your library or a playlist, even if you delete the local copy from your computer.
Basically, if you have a local copy of a track on your computer, Spotify should normally play from that copy, and then if it doesn't have access to it, it plays from the matched copy in the catalog.
If a song has been matched, it should behave just like a standard Spotify track, meaning the information, such as artist, album, etc., have white text, and the album and artist can be clicked on to go to Spotify's artist or album page. If they are not successfully matched, they will be grayed out, and I normally see a "download icon" that indicates it's only a local file.
In my experience, Spotify has generally been pretty good about matching tracks, but sometimes the metadata may vary in some way that prevents this from happening, and I'll usually end up just looking up a good copy through Spotify search instead of relying on my local copy, as it just tends to be easier to work around that way.
I know this was a lengthy response, but I hope all of this info helps you out! Thanks again, and let me know if any more questions about working with your local tracks!
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