Does Siri become pope? Siri becoming Pope has long been a Trad wet dream. He snaps his papal fingers and Shazam the Second Vatican Council is voided and overnight the Catholic Church becomes exactly like it was in the late 50's. Well no that is not going to happen but he is going to move the Church in a more conservative direction on the theological, political and disciplinary axes
Does Benelli becomes pope? On the theological axis he is a moderate but on the disciplinary axis he is an authoritarian on the discipline axis. Within a nanosecond he excommunicates Lefebvre but after that his main targets will be the likes of Kung and Curran
Does Wojtyla become pope? He almost certainly does not take that stupid double regnal name. Does advancing his papacy create butterflies. Maybe no assassination attack or then maybe it succeeds
These really are the questions that must be asked. Who becomes Pope?
The August 1978 conclave was, by all accounts, a deadlock between Siri and Benelli. Neither could assemble sufficient votes to reach the threshold, so Luciani ended up selected as a compromise. Presumably, if Luciani isn't available, and there is no other POD, you need another acceptable compromise candidate. Yes, it *could* have been Wojtyla - there was interest in him already, and he allegedly received a few votes - but I think the College was much less ready to take such a risk in August; they needed the shock of Luciani's sudden death to entertain such an unconventional (non-Italian) candidacy. Lord Grattan suggests Pignedoli, but I think he was too associated with Paul VI, too liberal to gain enough conservative votes in the conclave.
There's an obvious POD if we look past August to the October conclave, where Siri made the mistake of granting an embargoed interview to Italian journalist Gianni Licheri in which he made critical comments about Vatican II, only to have Licheri's article publish before the conclave, damaging his chances...if that interview doesn't happen, Siri has a better shot, though not a slam dunk. But that doesn't help us with the August conclave.
A Siri or Benelli pontificate would have a much greater likelihood of provoking schism for the reasons Tom gives - Siri really would have tried some kind of rollback of conciliar changes, which might have pushed some progressive prelates over the edge, while Benelli was sufficiently autocratic and abrasive to make plenty of enemies. The even bigger butterflies are those involving the Cold War, once John Paul II's enormous impact on Poland is taken away. It's hard to underestimate the impact that that nine day trip had on Solidarity's growth and success, with all that followed from it. After June 1979, the only thing that kept the communists in power was armed force, and the threat of its use, especially in Soviet form.