I would say that the genius of Zarathushtra is not that he proposed any dogmas but rather resisted that temptation and instead proposed the metadogmas within which a society can prosper and minds can become enlightened and strengthen each other.
This is what Mazdayasna is all about!!!
The problems that some have in translating some of the Gathic terms such as Ahura is due to three major reasons;
1- we forget that Gathic is almost as old as Sanskrit but more misunderstood.
2- we ignore the metaphorical differences in meaning of words which are similar but have different uses.
3- and most important of all, we forget that Gathic is not Avestan.
Also as you mentioned this was the dialogue between Zarathushtra and Ahura Mazda, mainly Mazda, that caused this philosophy to remain growing, flexible and adaptable.., that we now have the right to discuss and criticize it... had there been any answer provided, we should have zipped up our mouths and abided with all those answers... there is no answer or scientific hypothesis about the position of Earth in the universe... there is no prophecy about the Final Day... there is no flimsy tidings about life after death and the beautiful chicks in Heaven... it is all questions left to be answered...
Had Zarathushtra answered those questions he could have retarded the progress of minds for hundreds of years in the whole Iranian Plateau... there wouldn't be any Emanation Theory by Plato.., there wouldn't be any bright minded Spinoza... and still we are trying to answer those questions because this philosophy is an ever moving progressive one unlike the old, static and stale Abrahamic theologies.