I posted the excerpt because, for me at least, it clarifies the statement HHDL made which was quoted at the beginning of this thread. Mahamudra and Dzokchen have many similarities, but I think that 3rd Karmapa's prayer spells things out more completely and yet more succinctly than the original quote. Both Mahamudra and Dzokchen are less concerned with creating a system of assertions, than they are in facilitating practice and the birth of experience.
From that point of view, I maintain that all sentient beings have the inherent Buddha Nature, from the POV of conceptual discussion and dualism.
All sentient beings possess the "potential." This nature or potential is unconditioned, and therefore not subject to arising or ceasing--both of which are concepts, relative to conditioned phenomena and relative truth. But saying all of this is still within the realm of superimposition, by virtue of the very fact of using language and having discussion in the first place. In reality, one has to remain free of superimposition and denial, in practice. In my opinion, it is good to understand the Via Positiva in this particular case, as well as the Via Negativa. While there is a danger of reification, or, at worst, assertion of some sort of Atman or "Soul," by adopting only the Via Positiva, there are also some dangers in adopting only the system of non-affirming negation. As I understand it, the practical instructions of the Madhyamika maintain that, when one reaches confidence in the "very non-finding," one rests in that state or view...but still, there is "resting." This is very subtle, and I can't really "get at it" adequately I suppose....
I note again, these instructions are not for creating a "Philosophical System" but for facilitating practice. But,as the Karmapa wrote:
It is not existent - even the Victorious Ones do not see it,
It is not nonexistent - it is the basis of all samsara and nirvana,
It is not the contradiction of being both - it is the Middle Way path of union -
May we realize mind's essential reality, free from extremes.