Honestly, what is he expecting people to respond with when, in a conversation about cost of living/education, he states his daughter has just shy of one million to set her up?
I've sat here and tried to think of a non-offensive response, and all I can really come up with is 'Oh right'. I wouldn't know how the fuck to respond, OP.
Stating you've got nearly a million sat there for your child, so you never have to worry about finances for them, is a boast, regardless of how it came to be. Personally I'd never tell anyone how much money I had set aside, it's about as crass as your comment was taken. Honestly again, what the fuck response was he expecting from folk?
When I read the comment you made, I initially frowned - yes it's not the best response, but honestly, reading the thread again, I can see why you said what you did, it's clear the comment was about money, not the loss. She is lucky she never has to worry about affording education and buying a house. She is also very unlucky her mother passed away.
From the sounds of it as well, the mother passed away a long time ago. I get that grief is painful, and doesn't just go away - but after what I imagine is years, people should have accepted it enough that a thoughtless comment doesn't have grown adults running in circles to give you such a hard time, and they can accept it was a statement in a difficult conversation where the person making it probably didn't have much else to say, who would?
Honestly, loss is horrible, but why do people have to tiptoe around folk who've ever lost someone (which is a lot of people) years later, it's a bit ridiculous.
You've apologised, which is good. Don't apologise again. If any of the others decide to continue messaging you about it, either ignore them or tell them firmly, you have recognised it was a mistake, not meant to be harmful, you've apologised which is all you can do, and the matter is closed.