I don't know how Zoroaster defined 'prophet' and frankly I don't see that it matters given that Zoroastrianism doesn't seem to obsess over what makes a person a prophet and what doesn't; at least not to the extent you do. Why do you even care anyway? You've already written Zoroastrianism off as spiritually dead just to make your own beliefs look good. Perhaps if you spent more time focussing on the message rather than the messenger you'd see why this isn't the case.
I quote here from Yasna:
Yasna
13.
INVOCATIONS AND DEDICATIONS.
Y13.1-6 and the beginning of 7 is in the Old Avesta dialect.
1. I address (my invocation to) Ahura Mazda. And I invoke (among guardian beings) the chief of the house-lord, and the chief of the Vis-lord, and the chief of the Zantu-lord. And I invoke the chief of the province-lord. And the chief of women I invoke, the Mazdayasnian Faith, the blessed and good Parendi, her who is the holy one of human-kind. And I invoke this (holy) earth which bears us.
2. And I invoke the friendly and most helpful person's lord, the Fire of Ahura Mazda, and also the most energetic lords of holy men, those who are most strenuous in their care of cattle and the fields, and the chief of the thrifty tiller of the earth. And I invoke the steady settler of sanctity, (and) the chief of the charioteer.
3. And I invoke the chief of the fire-priest by means of the most imposing sciences of the Mazdayasnian Faith. And I invoke the chief of the Atharvan, and his pupils I invoke; yea, the lords of each of them. I invoke these lords, and I summon the Bountiful Immortals here,
and the Prophets who shall serve us, the wisest as they are, the most scrupulous in their exactness (as) they utter words (of doctrine and of service), the most devoted (to their duties likewise), and the most glorious in their thoughts(?). And I invoke the most imposing forces of the Mazdayasnian Faith, and the fire-priests I invoke, and the charioteers, the warriors, and the thrifty tillers of the soil.
4. And to You, O Ye Bountiful Immortals! Ye who rule aright, and dispose (of all) aright, I offer the flesh of my very frame, and all the blessings of my life.
http://avesta.org/yasna/yasna.htm
So Prophets were important to Zoroaster.
I request one to please read the above verses and let us know the definition of the prophets that fits in the context of the verse/s.
Regards
