1) We know about the opening phases of the death process from near-death experiences, which have been going on as long as our species is around -- one of Plato's dialogues describes a classic NDE, for example. We know about the rest of it partly from people who return to incarnation with clear memories, partly from the experiences of shamans and visionaries who have encountered the souls of the dead, partly from interactions with gods and other nonhuman spirits of high degree, and partly from apparitions of the dead to the living -- as I noted, some souls can get in touch with living people for one reason or another. None of these channels of communication are foolproof, but all of them tend to tell a consistent story.

2) Exactly. You can do the work now or you can do it later, and the more of it you do now, the easier of a time you'll have when you have to tackle all of it.