I believe ‘after all’ to be a shortening of ‘All things have been regarded!’ In an absolute sense, after it’s a predicate on all to be true and all things decided to be true. In the examples given “I’m sorry, but we’ve decided not to come after all” is “I’m sorry, but we’ve decided not to come after all things have been considered” and after they have looked at everything going on and considered them, they have decided not to come.

That’s sad.

I believe ‘after all’ to be a shortening of ‘All things have been regarded!’ In an absolute sense, after it’s a predicate on all to be true and all things decided to be true. In the examples given “I’m sorry, but we’ve decided not to come after all” is “I’m sorry, but we’ve decided not to come after all things have been considered” and after they have looked at everything going on and considered them, they have decided not to come.

That’s sad.