*spoiler*
if you die in a dream you wake up but they were sedated so they'd go to limbo but he was already in limbo so i don't think the dying in the van mattered at all.
Then he and the japanese guy shot each other in limbo to wake each other up in real life (the sedative probably wore off by then? or maybe because they were already in limbo it could work, but either way this is a proven method for getting out of limbo because he and his wife laid on the tracks to escape the first time.)
The very end couldn't have been in limbo because there was too much going on, limbo isn't an actual dream it's empty space and the only things there are what you create. The world in limbo was much different than the dream world or reality. It is suspicious that his kids hadn't aged but we aren't told how long he has been out of the country since his wife's death.
Just some thoughts.