The thing is, while the murders of Rizzuto and his allies seemed to be very well plannend and carried out with extreme efficiency, Montagna's murder certainly was not. This means that it was probably carried out by another group.

If the assumptions about that supposed mob meeting are true, I'm considering two possible scenarios:

1. Montagna could have been involved in a dispute with Montreal AND Ontario mafiosi, and his murder could therefore have been ordered by Ontario and carried out by local mobsters.

Or,

2. Montagna AND Ontario mafiosi became involved in a dispute with Montreal mafiosi which suggests a possible break in that coalition.

Both are equally plausible IMO. But I'm tending to believe more in the second scenario. If Montagna knew he had no more allies, he would have likely been more warry to meet with other gangsters, possibly even unarmed and without a bodyguard. Montagna's trips to Ontario, and considering that the first murders were most likely organized by Hamilton, suggest he was allied with Ontario mobsters.