Lana, I agree with you about Tom. At best, he's on borrowed time, but since Tom advised Tessio that he couldn't come either, that tells me that he was due to take the fateful ride with Michael that day. Tessio planned to take out Tom as well, and for good reason: like you said, Tom's loyalty to the Corleones was absolute, and Tessio would believe that there's no way to flip Tom. Additionally, he may or may not have relished the idea of a non Sicilian in such a high position in what would soon be his Family,

In my alternate scenario, Tessio would have had a meet arranged with both Michael and Barzini, with each believing the other requested it. In the movie and book, it's questionable as to whether there was an actual meet scheduled; if there was, I don't think Tessio let Barzini in on it being a setup to whack Michael (and Barzini himself, if my theory holds water). If Barzini did know it was a setup and Michael found Tessio out as a traitor, surely Barzini wouldn't be running around out in the open for Neri to take him out on the steps like that.

As for Fredo and Anthony, I don't think they're of concern to Barzini or Tessio. Anthony is just a kid (doing anything to him would be infamita) and Fredo is out west at the time in Vegas; neither would be a threat to Barzini or Tessio.