Paradoxically, I think the so-called "post national" culture idea is a national culture.
No it's not. It's saying "we don't have a national identity of significance, we're multicultural". The Liberal government doesn't push much of any national cultural or identity, they push multiculturalism and diversity.
If you want people to feel united then nurture a national identity/culture where everyone feels they belong. People in this country don't know very much about Canada's history, that would be a great start.