Saying a big 'Fuck this!' to the WMG for a moment, let's have a look at Gensokyo afterlife for a moment, mostly on Heaven since that gets absolutely NO focus in Touhou whatsoever.
It comes off to me as odd that, given how spiritual Touhou can be at some points, all we really ever get in terms of the afterlife are Hell-ish elements, Higan and 'Oh people die and become ghosts. The End'. I do NOT see Celestials as beings of heavenly origin and no one else should (the world they come from is stated to not be heaven in any sense, rather an exclusive club, if you will, where they hang out).
Let's begin with what we have so far, Sariel. She is by far the ONLY character that is truly from Heaven and we don't get much as to what her motives were in HtRP, or what her past was (as per the usual with PC98 characters). But let's stop and salvage what we can of what is from HRtP to peice things together on our own. First, her location; a Shrine, why is it that an Angel resides in a shrine? Could this be a clue as to how things work in Heaven? It very well could be...
Given that one and only clue, I speculate that, in Heaven, Angels are actually dead miko. Since, miko themselves can comminucate with the gods who dwell in heaven, this means that they are thus granted 'special rights' when they die just for being closely tied to Gods. So, when a miko dies or is killed, these 'rights' are presented to the Yama upon her death. The Yama allow her to ascend to Heaven, free of judging, and there the miko in question becomes an Angel and comes to work along side the Gods/Goddesses she communicated with. HOWever there's a huge catch to this; in order to ascend, a miko MUST remain pious and keep in close contact with the gods, via having one at her respective shrine. Otherwise, her access pass to Heaven is pretty much fragged.
Reimu's Angel rights, however, are void and she's doomed to be judged at the full extent of Higan Law. This is given credit when one looks back at what Shiki said upon commenting on her afterlife.
In contrast, Sanae has a clear shot of becoming an Angel given how much of a good girl she is.