Only hope here is that the writers are playing with the people online that have been pushing the 'dead mother' theory over the years. Maybe she had an illness (say, cancer) in 2024 and there was a chance she wasn't going to make it, but eventually got better/cured/remission, so can still be there alive in 'present day'. lets them feed the fire a little, but doesn't put such a wet blanket on the series. They thought she might be dying, went to the inn to get away and spend time together, but don't have to kill her.
It being HIS mother that died just doesn't fit. They could have said what kind of parent misses their kid's wedding, but picked mother and daughter there, so that's the implication.
That, and the kids don't seem sad enough for the story to be after the mother's death. Plus, super inappropriate to spend most of the 9 year story about your dead mother by telling the kids about how you used to screw their Aunt Robin plus half the floozies in NYC. Not impossible they're going that route, but seems a bit off. Also, the son's reaction to Ted in the pilot episode when he says he's going to tell them the story is "is this going to take long?" which is a pretty cold/dick thing to say if your mother is dead and your dad wants to tell you a story about her...
Could be Robin that's dead, and references still fit. And why the story has so much Robin and not so much Mother. I know, path he took to get there, but little much of him nailing their aunt to be ok there. And if Robin somehow got pregnant and had a daughter, but died by 2030, she could be the one missing her daughter's wedding.
Or Ted has Alzheimer's. That's why he's telling the stories while he can, why the stories don't always line up right, and why the Mother told him to live for the future and not so much about the past.
I dunno, lots of ways to go, but hope it's just the writers teasing the Dead Mother crowd before moving away from it...