I see you're point of view, but I could imagine that William would still want an alliance with the Hohenstaufens and/or make the Hohenstaufens give up their claim on Southern Italy.
I can also see Tancred not having exactly the same support he did IOTL, especially if Henry of Swabia doesn't have a claim via Constance, but there might be at least one noble who might prefer having an adult on the throne instead of a regency under Joan, similar to what happened to Alexios II and Ladislaus III of Hungary. Plus, Tancred had already rebelled against the crown before to try and depose William I.
Thinking about brides for Bohemond, I was thinking either Eirene Angelina (she married Tancred's eldest son IOTL before marrying Philip of Swabia) or either Constance of Aragon (wife of Emeric of Hungary and first wife of Emperor Frederick II) or Eleanor of Aragon (sixth wife of Raymond VI of Toulouse.
Speaking of Raymond VI, with Joan in Sicily, he might not repudiate his marriage to Bourguigne de Lusignan. And if he doesn't have any male issue without marrying Joan, the County of Toulouse might not pass to the French crown, depending on who his daughter, Constance, marries/remarries, which could very well be Philip Augustus.
There could be a thought - Philip Augustus remarries to Constance of Toulouse instead of Ingeborg of Denmark, while Ingeborg marries Henry or Philip of Swabia.