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The descriptions for BG1 and BG2 (and IWD) say it includes the classic version, but is this a separate download/installation that can be done without the EE? So I can install the original game and use my own mods?

This question / problem has been solved by dtgreeneimage
It is a separate download.

In fact, when you get the EE, the original edition isn't automatically added to your library; you have to go to the EE in the library, choose More -> Serial Keys, and use the link there to actually add the original version to your library.

It is possible to, instead, give the link to someone else, who can then add it to her library, but the key can only be used once.