This is not day of travel IRROPS.

Indeed NH will only have initial access to other award space on AC and will need to ask -- I stress ask -- AC to open award space. AC may not do so and there is nothing NH can do to force AC to do so.

In that event, NH will attempt other reroutes or refund the ticket.

To the contrary on the day of travel. That is indeed AC's to handle and AC will rebook into any available space. If this were to occur the day before travel, I would nonetheless call AC and see if it will help. If not and NH is not helpfui, I would wait until the clock strikes midnight and call AC, it now being an AC problem.