JohnRammy, you're just wrong--Luminous Mind is mentioned in the Suttas and Nikayas, and in the Tathagatabarbha Sutras like Surangama Sutra and Lankavatara Sutra...as well as is Maitreya/Asanga Shastras, etc. So, merely from the POV of it being "found" outside the Tantras and Dohas, you're wrong.
As Karma Dorje said earlier:
Of course there is great identity between the tathagathagarbha sutras, the tantras and the dohas of mahamudra: they describe the same underlying reality of luminous emptiness. We need not assume that these are all of a single piece historically.
I have granted that "Mahamudra" as a term is found in the Tantras (and in the Dohas, which are mainly tantric in nature), and not in the Sutras. But "Luminous Mind" and it's many synonyms is found in many places. You argued that Mikyo Dorje says the "luminous Mind" as found in Tathagatagarbha doctrine, and the Sutras in general, is different from this mind in the Tantras. So...if you are arguing that the "Luminous Nature of Mind" as found in Non-Tantric Material is different, essentially, than that found in the Dohas and Tantras, would you explain exactly in what way?
To quote from your currently favorite book, "The Center of the Sunlit Sky," p.62:
"In the sutras, the Tantric meaning is taught implicitly in a hidden manner, but the Sutric path does not operate with Buddha nature as it is taught in the Tantras."
The question, then, is what is the difference between Buddha Nature as taught in the Sutras, and as taught in the Tantras. Another question might be: is that which is taught implicitly, in a hidden manner, nonetheless "existent" there?