Please do not take anything I say personally - it can come off as crass and rude and I would simply say that I am passionate about Druid lore, and am therefore incredibly critical of it.
Alright. This is a negative from me.
As far as endgames go, I don't care one way or the other. I love ideas for this sort of thing and the concept itself I think is super interesting and not far off from what Ouity was thinking a year or two ago. But I take issue with several parts of this lore and its direction.
In no particular order:
The Aspects could care less what goes on in the mortal world between people. The Aspects were never designed to interact with players directly in any way. This is less of a jab at this lore, but previous precedents missed the mark in that regard. We hear from the Aspects about as much as we hear from God in the real world.
To suggest that a person would be denied re-entry because of actions on the earthly plane is to fundamentally not understand the nature of the Aspects (no pun intended).
I hate this. I'm sorry. I love the idea, an ancient Druidic Spirit bestowing powers to a lesser Druid as a boon, that's ******* awesome. But these powers for one thing I think are paltry and silly in many regards, and wholly unnecessary. Furthermore these abilities aren't in line with the Druid's own strengths either. I get that you want to add more abilities, but instead of doing that I would suggest allowing them to go beyond tier 5 in abilities, and bestow that strength to other druids, or even remove certain red lines from existing abilities in these heightened states. No need to continue re-inventing the wheel.
As I said earlier, unattunement after everything you have established with this lore would most definitely be impossible. Some have argued with me that because their soul is still attuned it can still be severed. But to that I say that at that point it would be far beyond the power of the Druids to sever such a connection. It would simply be too strong for their own abilities to touch. Furthermore I think it would take a very large amount of powerful Druids to permanently banish the spirit.
Perhaps a few druids could banish the spirit back to the fae, and then if enough tier 5 Druids gathered they could permanently do it if at the original guardian circle?
Finally I can't help but think this simply is an echo of Soul Trees that add extra powers. I mean here are TL;DR versions of both Soul trees and Guardian Spirits in their most simplistic forms:
Old Druids sacrifice their life to transfer their soul
If killed, cannot return for some time
Druid is immortal now unless unattuned
etc
All that said, I like the concept, and I also really enjoy the writing! You did a good job writing the lore, but to me I don't think it's something overall that is not suited for the Druids. With a direction rework it would be a great boon though.
And since you can't complain without offering solutions, here are my own suggestions here.
Rework the Unattunement portion
Undo most of the abilities to interact with the world physically. Let the sacrifice of body mean something. Let them manifest in spirit form, let them even use their powers but limit their activities. That would give the ability to grant power to other Druids even more significance.
On the subject of powers, don't kill yourself trying to come up with new powers. Let it enhance past tier 5, and even remove certain red lines for a temporary span of time.
Remove the bit about the Aspects barring them from re-entering for engaging in political stuff. I apologize for the disrespect, but that's really dumb.
Expand on the corruption section, a villain spirit would be most interesting. Don't make the corrupted spirit die after a certain period of time, it undermines how devastating a corrupted spirit has the potential to be. Make it so they must be banished back to the Fae in order to be restored or something.
EDIT: Spirit manifestation - I really don't see any reason to allow them to manifest as whatever they want. It should be as they left the world, my own personal opinion.