Subject: On the subject of SPN angels... (SPOILERS for SERIES))
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Posted on: 2013-05-29 00:51:00 UTC
Answers to your questions, as far as I know, in order:
1.0) I don't think it really relates that way; angels in Supernatural were never human to begin with, so it's more like they're reverting to the form that they take-- their human vessels --when they manifest on Earth. Demons were once humans who had their souls marred and broken in some way, either through some sort of demonic crossroads deal or other acts of extreme violence.
1.5) We don't know; I don't think that's ever happened before.
2.0) Again, we don't know. The only angel we saw that ripped out her Grace and was reborn as a human was Anna (Anael), and she, as far as we know, never had kids. Then again, being stuck in an asylum probably didn't help that matter.
2.1) Unknown on the babies thing; Anna only started hearing 'angel radio' (Aka, the grand celestial chatter of bored angels everywhere) when another angel, Castiel, rescued Dean Winchester from Hell and proclaimed that he was saved, opening her psychic ears to what was going on.
2.2) Supernatural and religion. Right. That's... ridiculously complicated, actually. I mean, yeah, they play off the Judeo-Christian mythologies quite a bit inside the show, but it's more like cherry picked bits and pieces here and there rather than following a straight up recognizable dogma. There's also the fact that they throw in bits and pieces of other religions and mythologies-- reference 'Hammer of the Gods', episode 5.19, if you want more of an idea --all the time, so that's not much help.
3.0) It's a little of both. The tree is a physical marker of the Grace, but it doesn't disappear once the Grace is retrieved by someone.
3.1) Anna's Grace was a very large tree, but we've never seen anyone else's, so it's just a lone sample in canon. In fic, though, I've read a story where an archangel's Grace (that story's canon followed the seven archangel theory rather than the show's canon four) was actually the cause of the Great Barrier Reef. Grace = life force, so it makes sense that someone more powerful than a 'normal' angel would have a greater impact on nature.
3.2) Unknown.
3.3) Again, unknown, though I suspect the Grace itself would probably act as some sort of protective countermeasure; maybe its very presence would somehow influence people to leave said tree/organism alone?
4.0) Let's see... In canon, angels have lost their Grace due to the following:
a) Dying [which can be done only via another angel's sword or explosive means]
b) Angels abandoning Heaven due to the Judeo-Christian Apocalypse in an alternate timeline, leaving 2014!Castiel to slowly lose his powers
c) Forceful removal
d) A ritual closing off Heaven from angels (see the Season 8 season finale for more details)
4.1) Actually, we don't know much about the angelic hierarchy in the SPN 'verse, save for bits and pieces here and there. Obviously, the four archangels are the powerhouses of the whole thing (though that doesn't mean that they do a good job at it [those who are active in Heaven for any length of time, anyways]). We also know that most of the angels are arranged into garrisons, or groups of warriors. I say most, since I don't know about the cupids, or if the reapers can be classified as angels or just celestial beings. The garrisons are headed up by captains; Castiel was the captain of the garrison that broke into Hell and got Dean back.
Hope that helps!