Is there a way for me to utilize the watch list in Sandbox mode? Any time I do, I get a "you must be logged into a team..." message. I have my team set to Human - am I missing something?

I'm not sure it works well without being in multiplayer mode where you can "log in" to a team or career mode logged in as your own team.

I started a Multiplayer for kicks to see if I could use the watch list and the "add to watch list" button is no longer on player cards?

You'd have to be logged in to a team.