I find it hard to believe USair would reroute with Elal, but glad they worked things out at the end
+1
Air Canad=Star Alliance Partner
And if it was a reroute with LY, JFK to YYZ would be with AA as LY is a partners (for now). Not Delta.
-1
As Chaikel said, when everything hits the fan, you can ask for pretty much any flight on any carrier. If you get resistance, you just haven't climbed high enough on the totem pole yet.
During a recent snowstorm, my parents' LGA-MIA flight on AA got cancelled and rebooked twice by the automated system, the second time for a day or two later. Wanting to help them get from snow to sun sooner, I called AA and got them rebooked - first on Delta, and then on United, and then back on AA again. So that's all 3 alliances on an Avios ticked.