Great subject input.
I would like to share the reason that Dorje Shugden is a Buddha because I decide he is a Buddha.
It is said that if we view our Guru as a Buddha we receive the blessings of a Buddha. This statement speaks volumes about how our subjective reality is created by our own mind.
First we establish what a Buddha is and how it is possible for such a being to exist. All sorts of things help us establish these things - logic, experience, anecdotes, scripture etc. These things give rise to a conclusive decision in our own mind that Buddhas exist.
Also, it is said we do not know for sure who is actually Buddha unless we ourselves are a Buddha. So, we can give all sorts of reasons why Dorje Shugden is a Buddha but these are inferential cognizers realizing a phenomena indirectly. By deepening realization we eventually come to know it directly. These are the two types of proof. But really, based on understanding the benefits that come from seeing the Guru as Buddha, we decide to hold that perception. This applies to our view of other people and indeed all phenomena. Through generating and becoming familiar with this view, we come to rest non-conceptually in the meaning of this view, in its nature, which is none other than the conventional and ultimate nature of our own mind that is not separate from the Gurus mind - mahamudra.
There is a function is describing all the things you've shared, but I believe these things about the Protector are underpinned by the things that establish that Buddhas exist, and the two paraphrases I mentioned indicate that it boils down to our own subjective decision based on our belief.
If you believe Buddhas exist, and you decide someone difficult or a difficult circumstance is a Buddha, you will receive the blessings of a Buddha. This is a very profound training and is the reason that Guru Yoga is the quick path. Therefore, Dorje Shugden is a Buddha because you've decided he is. And boy o boy, tell me those blessings aren't intense!