After spending an hour (twice) setting up all the logos and colors of a league I get an error when trying to load the league again:

Error
Error loading league: Sequence contains no matching element.

EDIT: Actually I think I figured this out. After creating both leagues I went to change teams but chose a blank slot in hopes I didn't have a team and could just observe (it worked in testing, but maybe I never quit the game and reopened?) So my next question is this: if this is in fact the cause is there an easy way to edit a file to assign a team to myself so the league will load? Otherwise I'll need to recreate a new league and go through the logo and coloring process again

Also, can I just observe a league? Or do I NEED to GM a team?

You can check in the mod forum to see how they set up logos and team names using mods instead of doing it manually in the game.

Other than that, if you pick sandbox you can run all, or none, of the teams if you wish to just observe.