After trawling through volumes of dictionaries, glossaries and texts, I've come pretty well to the conclusion of this line of enquiry. I'm sure that specialists in the field of Proto-Indo European culture would be able to extend the study further but this, I guess if for another day. So, what are my conclusions:
- Whilst the Māra as found within Buddhist mythology can be demonstrated to by a modification of the Indra/Namuci myth, the origins of the term are derived from other sources.
- The Buddha's initial quest was to find the deathless condition, i.e. amṛta. He overcome death in the form of Māra. but he does not eradicate Māra. The Buddha does not become Māra-mārāka, 'the killer of death'.
- Māra does not come as pain, old age, sickness and death itself but as a plaguing spirit which seeks to 'rub-away' at the emotional stability of the Buddha.
- There are other roots from which a word Māra could plausibly be obtained: i.e. mṛd and not mṛt.
- Māra does not seek to destroy the Buddha or cut short his life, but to 'hinder' or 'thwart' the revelation of his teaching.
- Māra appears in the hypogogic sate of mind between full awakening and sleep. Māra, is then, a mare.
- Māra originates from within some pre-existing Proto-Indo European mythplex which is still reflected in extant European languages but becomes apotheosized in Buddhist subculture as Māra Deputra. Within Indo-Iranian mythology, the role of Māra is absorbed into Yakṣa mythplex whose origins are most likely founded within the mythologies of pre-aryan indigenous peoples.