Zoroaster or Zarathushtra was an Iranian prophet, not a god.
Not by the time of Justinian. Also, it's usually rendered Mithras, although, Mithran might be the accusative form. He was very tenuously based on the Indo-Iranian god (later "angel") Mithra (or Mitra).
I am not particularly fond of deva but it is better than a lame, made up word like aasimar.
(The Lord of the Flies reference was great, by the way. "Sucks to your aasimar!")
Yeah, I remember that part of the Chanson de Roland
where Roland waxes verbose about the Forms in the midst of battling the Islamic Saracens. And it is well known that Galahad always carried a copy of the Laws in his saddle bag as he searched for the Holy Grail.