Avestan ? Please, please, please

I'd just like you all to know that when I ask these random questions,
there really is a reason... and hopefully in the next month or two all
this research should pan out with a few articles being posted to the
PIE religion list.

Having said that, I am hoping you all can help me out yet again.

I am researching a group of related items and am trying to understand
the roots and etymologies of their names:
1) The Zoroastrian deity Tishtrya
2) The supposed Vedic correlate Tishya
3) How Tishtrya produces the later name Tir for the same deity
4) Tishtrya's foe the Zoroastrian Pairika Duzyairya (aka Huyairya )

A great many thanks for any help you can provide,
Aydan