As usual TB, your analysis is pretty good. Yes, we don't know when Barzini conceived of murdering Michael. After reviewing the transcript, I doubt Michael's murder was planned after the Don's meeting. The transcript does not provide a timeline, but the transcript sequence has Tommasino visiting Michael right after Vito visits with with Bonasera which indicates that Sonny's death and the attempt on Michael happened right after and before the Don's meeting.