Methinks that your boss is concerned about his status. That's just me looking at this from a distance. It might have been a good or a bad idea to talk to HR - or both at the same time. That's life.

One way to approach it is to assure him that this just happened because he was out of the office and you will never go "over his head" again... that takes care of his status concern. He will probably calm down.

It's also the job of the senior boss to take care of your line manager's concerns - involve him in the ongoing discussion - smooth his ruffled feathers, make sure that he doesn't feel side-lined.

Sounds a bit like your anxiety needs addressing internally. Hey, everyone makes mistakes... I bet your boss and his boss have made a few mistakes in their lives, and in the future you may manage staff who occasionally put you in a difficult position yourself.

You sound very professional.