one25six and 3slips, you're both wrong on the last one.
The rules say 6 miles behind a heavy FROM WHERE THE HEAVY IS RIGHT NOW, not from where he was 2 minutes ago!
If you had a light 6nm in trail behind the heavy and at the same speed, you'd climb or descend him through the heavy's altitude without worrying. The C402 in the illustration is going in the opposite direction and the point at which you'd climb or descend him were he in trail is NOT A+6!!! It's 6nm from where the heavy is right now!!! That will most likely be about A+1.5 or so. You don't need to provide extra separation just because they're going in opposite directions.