Curious if some of you more savvy tech types can explain this to me. I have received the multi-player league files from the commissioner, and plunked them into my "League" sub-directory. Do these overwrite anything in terms of single-player? Can anyone download these files and view them without impacting their other leagues/games?

Multi-player league files do not effect single player.
Your single player file is in your Universe directory, and of course multi-player is in Leagues.

Unzip your multi-player files to your Leagues directory, and you will be fine.

When you want to play the multi-player team, open the multi-player icon (the head, or person face) and it will look to the League folder.
For single player, just open as usual.