Wouldn't Robert and Nathan see each other at Maggie's funeral?
If Robert's wife Kate knew Maggie and Robert's family was caroling at Maggie's home at the moment she passes, wouldn't they go to her funeral? And I assume Robert would attend it to comfort Kate. It would only be logical to conclude that Robert and Nathan crossed paths there instead of 15 years later by random chance at their mothers' graves on Christmas and Robert did not recognize Nathan until he drove off. That's just really corny writing.
If you woke up this morning, it's because Chuck Norris spared you!