I would tend to say
Sara (and all of us) have a lot to think about now.
...but it occurs to me that if I remove the parenthetical it's clearly incorrect. Is it proper to say
Sara (and all of us) has a lot to think about now.
...that sounds really strange to my ear.
Also, does it change the rules in any way if I use an em dash, e.g.
Sara–and all of us–have a lot to think about now.