I wish to do this itin:
HKG-xNYC-YUL-NYC-CDG-HKG
I only stated NYC because I wish to use CX nonstop to JFK and directly connect to Montreal where I spend a few days, then a few days in NYC.
Is that a valid itin?
Perfectly valid
If you wish you can stopover in New York before and after Montreal - there is no restriction regarding multiple stopovers in the same city
As pointed out by
Lux you cannot include Openskies in your itinerary, so your JFK-CDG flight will have to be on AA