I have just done as I described, and downloaded e-mails locally from one folder locally.
Now, when I open that saved folder, I have within that folder a index.html file and a messages folder - if I open that messages folder, the individual e-mails are in there in eml format.
It may be slightly different in the OS you're using (I'm using Linux Mint), but if I right-click one of those messages with the eml extension, to open with Thunderbird, it does exactly that.
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