This has always been in the back of my mind as well when I have award tickets involving partners. I think once the ticket is issued, there is a CoC there. What is the DOT rule if it were a revenue ticket issued by US with NH flight in it that subsequently got cancelled? Who has the responsibility to find the replacement flight?
flight cancellations happen. And this is acknowledged by the CoC. If your flight is cancelled often the airline that cancels will put you on another flight. Sometimes they muck this up because they only change their segment, and all your connecting flights now mismatch.
however at other times the airline canceling does nothing, then it is up to USDM to try and find alternative accommodation for you. They can either try different airlines, or, if you make out an exceptional case, they will sometimes speak directly with the other airline to figure something out (via the star alliance desk). If nothing can be worked out then you may have to take the full refund.
sticking solely to the 'I have a CoC' line may mean you end up with nothing unfortunately. So be proactive and see what else you can find.