Isn't it all just symbolic ?
It's a movie about a guy getting over his girlfriend ( who may have died or simply just dumped him) and moving on.
The zombies, the coming back from death, everything changing, life becoming a real mess, people bonding, I think it was all... not in his head but the way he felt.
I think it was just an allegory.
In the end in the car, it all felt normal because he got over beth and was going out with someone else.
And even if I'm wrong, it's a fun movie, dane dehaan rocks and so does aubrey plaza so no harm done.