Like the scenes that take place in Heaven, the concrete reality of Mike's afterlife visit to Jubal is ambiguous. Heinlein leaves us to decide for ourselves to what extent the scene functions as metaphor. Certainly we can accept as fact that Jubal decides to take an overdose of pills and that, as his consciousness is drifting, he vomits and saves his own life. But Heinlein intentionally narrates Mike's visitation very quickly, and in vague language to suggest that perhaps Jubal is hallucinating. The point is not that Jubal has lost his mind, but rather that it is inconsequential whether he is hallucinating. The Mike that exists within Jubal's mind now is as powerful and real as the Mike he had conversed with hours earlier.
A similar ambiguity applies to the final chapter in which Mike ascends to Heaven and begins his work as an archangel. Mike's continuing influence on the happenings of planet Earth can be seen literally as the toil of an angel in Heaven or metaphorically as a powerful leader's continuing influence in the hearts and minds of the populace. This duality of the angels' roles helps to explain why such apparently impure con artists such as Foster and Digby could have found a place in God's employ in Heaven. Because legions of followers have accepted Foster and Digby as their holy leaders, and their teachings remain long after they die, then they indeed have a continuing "Heavenly" influence on the Earth. Mike joins their company, as well as that of Jesus, Mohammed, and history's other great prophets. Or, in Martian terms, Mike becomes one of the Old Ones, the spirits who rule over the planet not in spite of the fact that their bodies have died, but because of it.