Purgatory is an afterlife and from the sounds of it, a not very pleasant place. I think it's supposed to resemble Hell, which makes sense with the Dantean imagery they like to go for. The whole Midway upon the journey of our life / I found myself within a forest dark /For the straightforward pathway had been lost thing, you know?
But it isn't Hell. Or at least, it isn't the Hell that we saw for Dean and Sam - no endless void with only chains, no endless fires. It's a gloomy, dark forest inhabited by glowing red eyes - presumably the souls of the dead monsters we've heard about. It's not a place where humans are supposed to end up and what we've seen of it isn't pleasant, but I don't think it's supposed to be a direct analogy to Hell. It's the afterlife for monsters - or at least, for non-humans. And demons and angels both count as non-humans.
As for your other questions, IDK. John didn't seem to be a demon when he crawled out of Hell after a year. Dean didn't seem to be a demon either despite 40 years downstairs. He might have been on his way to becoming one, but there's never been anything to indicate that he'd actually crossed over. Demons aren't human, but they used to be human. Just like angels can tear out their grace and become human, but they aren't themselves human. I see it as three different species, just linked together.
As for how they can climb out, it sounds like particularly powerful demons can go between Hell and Earth on their own (YED, Alistair, Lilith) while others (Ruby, Meg) have a much harder time with it once they're exorcised. Or maybe it's that exorcism is a binding ritual of some sort - it's not just that you're kicking their pass back to the pit, it's also that you're locking the door after them and they've got to be summoned to make it back.
I always thought that once topside, a demon could summon other demons but that until S1, it was rare that any would do so. Bobby even says something about that in Devil's Trap, about how there were maybe 1-2 possessions a year until S1, and then there were like a dozen of them, maybe more. I don't remember the exact numbers, but clearly there weren't a lot of demons until Azazel started the ball rolling and even then, it wasn't until he opened the Devil's Gate to Hell and released hundreds of them that demons became the threat that they are currently.
Re: John's and Mary's absence from heaven (small spoilers)
Purgatory is an afterlife and from the sounds of it, a not very pleasant place. I think it's supposed to resemble Hell, which makes sense with the Dantean imagery they like to go for. The whole Midway upon the journey of our life / I found myself within a forest dark /For the straightforward pathway had been lost thing, you know?
But it isn't Hell. Or at least, it isn't the Hell that we saw for Dean and Sam - no endless void with only chains, no endless fires. It's a gloomy, dark forest inhabited by glowing red eyes - presumably the souls of the dead monsters we've heard about. It's not a place where humans are supposed to end up and what we've seen of it isn't pleasant, but I don't think it's supposed to be a direct analogy to Hell. It's the afterlife for monsters - or at least, for non-humans. And demons and angels both count as non-humans.
As for your other questions, IDK. John didn't seem to be a demon when he crawled out of Hell after a year. Dean didn't seem to be a demon either despite 40 years downstairs. He might have been on his way to becoming one, but there's never been anything to indicate that he'd actually crossed over. Demons aren't human, but they used to be human. Just like angels can tear out their grace and become human, but they aren't themselves human. I see it as three different species, just linked together.
As for how they can climb out, it sounds like particularly powerful demons can go between Hell and Earth on their own (YED, Alistair, Lilith) while others (Ruby, Meg) have a much harder time with it once they're exorcised. Or maybe it's that exorcism is a binding ritual of some sort - it's not just that you're kicking their pass back to the pit, it's also that you're locking the door after them and they've got to be summoned to make it back.
I always thought that once topside, a demon could summon other demons but that until S1, it was rare that any would do so. Bobby even says something about that in Devil's Trap, about how there were maybe 1-2 possessions a year until S1, and then there were like a dozen of them, maybe more. I don't remember the exact numbers, but clearly there weren't a lot of demons until Azazel started the ball rolling and even then, it wasn't until he opened the Devil's Gate to Hell and released hundreds of them that demons became the threat that they are currently.