Again, I don’t agree with your analysis Gabriel. Very sorry.
I will reformulate somewhat your own words as follows:
It all rests on the assumption that any conceivable (non-Buddhist) attā or (non-Buddhist) attā-theor(ies) have at their center an attā that is possessed - my attā - in relation to the khandhas and the ayatanani. That is to say, that the atta is considered to be also the khandhas and the ayatanani, among the “whole” rest. (This is a generalisation though - for this is not true for the Samkhya philosophy, for instance).
While a Buddhist atta cannot be the khandhas and the ayatanani.
Again what exist and non-exist (arise and fade - live and die) are the khandhas and the ayatanani. Not the atta.
Buddha is not an annihilationist. (SN 22.47 - SN 12.15). He is not just concerned by the appropriated (“clinging”) khandhas, as in SN 22.47).
He does not refer only to the appropriated (“clinging”) khandhas; as the eternalists & annihilationists do.
There is an atta as a personal pronoun, and as a spiritual atta in Buddhism. It is just that it is not in the khandhas and the ayatanani. The personal pronoun atta cannot say of the latter “this is mine, this I am”.
So in a way, all atta’s theories are based on a “my atta” . But the non-Buddhist theories in the suttas, make it a universal Atta/atta encompassing a “whole” One - while Buddhism (and Samkhya, for instance), do not include the khandhas and the ayatanani in the big picture.
In Buddhism, the spiritual Big Self is outside paticcasamupada; if ever existing (and not relevant to the Teaching anyway) . And in Samkhya, it is outside prakriti. While in the rest of the theories, it is in the “whole” encompassing One.
In the meantime, once the citta has been liberated from the appropriated (“clinging”) khandhas (“this is mine”) , and from seing atta in the khandhas, or atta as khandhas, etc. (“I am”) - the atta as personal pronoun, dwells in the “deathless” - outside the kama & rupa loka. A spiritual atta, detached from matter and senses. Yet not completely realised.
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So, in @Erik_ODonnell 's diagram, the Buddhist atta, as personal pronoun should be seen in both the blue and white part. As a material atta in the former, and a spiritual atta in the latter.
Also the higher jhanas should be outside the blue circle. Idem for the arupa heavenly realms.
And lastly, the khandhas in the blue circle, refer to the khandhas in namarupa nidana and their consequential appropriated (“clinging”) khandhas by satta.
Not the ones that have to be gotten rid of in the higher jhanas - that is to say, viññāṇa nidana, and sankhara nidana. The non-manifested (anidassana) khandhas. The khandhas with no-thing.
Metta