Kerr hates ball-stoppers/surveyors, which Tatum is. Also, it made sense to go guard heavy against Serbia. I think against lesser teams you'll see him play, but against teams with a real chance of losing, you'll see his time limited
Embiid needs to sit at this point. There was merit to seeing if you could play him into good form, but he's been a net negative against the starters of other teams and he doesn't really fit how this team plays. AD and Bam rotate with the option to go big by playing both or small with Lebron and KD. Embiid can match up with true big centers when you play them
Regarding who starts/closes, keep in mind Holiday needs to be in those lineups. He's the best on-ball defender on the team so he's guarding the other team's best perimeter player, and he's shown to be an excellent catch-and-shoot spacer, connector, and cutter