Well, I'm not really sure that anyone has real "knowledge" about a realm that's supposed to exist outside of human perception, but I can give you my thoughts as an atheist, and the doctrine of the Catholic Church, to which I used to belong.
My major evidence for atheism (well, materialism really, in the philosophical sense of the word), is that everything around us contradicts the idea that humankind is special, is different from the rest of the universe, with a unique purpose and a unique destiny. There's just no evidence for a world built around humanity's needs or experience at all. Each of us feels as if we ought to be very special, that our suffering should be minimal or at least meaningful, but then the woodchuck digging up my front yard probably feels the same way. For all I know, so does the grass. Maybe so do bacteria. Why should the entire universe exist purely so humans can prove their spiritual mettle in it, with the sole aim of dying and going somewhere different and better? I think the woodchuck would be offended at the idea. There's also no need for an invisible world to exist at all. Everything runs just fine without it. The only thing that "requires" such a thing is us, in that it upsets us that we are not immortal or special. Totally understandable that we would feel that way, but feeling as if something ought to be true does not imply that it is.
I suppose none of that precludes any possible spiritual belief system, but it does paint the existence of devils who want human souls in an extremely doubtful light.
Speaking as a onetime faithful Catholic, I can give you more specific answers from the perspective of that religious denomination. 1) Yes, selling your soul to the devil would constitute spiritual suicide. 2) You cannot opt in or out of spiritual planes. They simply exist, and you have to deal with that as best you can. 3) Your experience will never and can never be repeated. There's no such thing as reincarnation, and time is an arrow, not a circle. The past is finished forever. The future holds the rest of your earthly lifespan, which will be pretty brief, in the scheme of things, and then a spiritual eternity. It is up to you whether you choose an eternity of suffering in hell, or an eternity of bliss in heaven (probably with an unpleasant stopover in purgatory on the way).
As I said, there's no evidence for any of that being true, but that is the sort of thing that people who believe in souls and devils will tell you.
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