@fasquardon
1. Both of your objections of positions regarding he existence of the Zoroastrianism, can be given greater doubt than my views.
-The argument that the Gathas were written in a tongue that is ancient in its grammar and style is your first argument as to the antiquity of Zoroastrianism. This point, I would define as weak, simply by virtue that the concept of grammar and linguistic style can be preserved in older forms than the the current times. There are numerous examples of this. Today, I could compose a document in Classical Arabic from the 8th century and with practice, even in the Kufic script... Does this mean that said work is form the 8th century? No, but the language and style has been preserved and we have a memory of it remaining. A further example, in the Middle Ages, there was a song composed in Catalonia, called "Stella Splendens", in Medieval Latin; this song from the 14-15th century (1399-1401) yet its style, wording and such appear to be derived from the 11th century. Does this song thus and its surrounding works derive from this older period? No, rather it was simply composed by particularly conservative people and lyricists. A further example nearby the region of Iran, even in the time of the Arsacid empire, we have reliefs, depictions, saying and so forth in Akkadian that carry with it a conservative nature reserved for religious texts and general courtly language. Do these examples mean that they derive from the Babylonian Empire or the Assyrian Empire? No, it simply shows the propensity of humans to maintain a language for certain purposes whether poetically, religiously, politically, etc.
-Another point you made, is that the Gathas depict an environment that differs from Fars and correlates to the land of Ferghana. This is an even weaker point in my view, we know that humans have a propensity to imagine environments and discuss these in detail that are not within their existing lands. In the Bible, the book describes the Garden of Eden in ways that may not exist in the place that Jews existed, yet they conceived of this idea. We do not say, oh the Jews must have been residing in some sort of lush river valley, rather we say, that this is something that they have conjured as their reality that is not too different from what they see in home. It would be one thing, if the Gathas described glaciers and vast forests and animals found in only a certain place. Rather, it is describing Ferghana and the Oxus river valley, which is regions that the people of the time even knew of through trade and transit. Regardless, had Zoroastrianism existed in the Ferghana or Oxus, we would see the evidences of Zoroastrianism upon the Kushanshahs. What we find instead, is the Iranic subjects of the Kushan Empire, are being appealed to through the usages of:
Mithra: An Iranic god, which in Kushan coinage is associated with Apollo and often the figure is mixed between the two. Appears 20%
Wesho: The Iranic god of the winds and in Kushan coins, is often equated and accompanied by Hercules. Appears 6-7% (Hercules and Wesho) (during the reign of earlier Kushan emperors, such as Kanishka I the percentage is much higher and is approximately at 30%, while Mithra is 18%, Nanahita is 21%, Atsho is 4%, Mao is 15%, Buddha 1%, Helios 2%, Zeus 2%, etc...)
Mao/Men: The Iranic and Anatolian god of the moon. Appears 12%
Nanahita: Sogdian goddess of water and springs. Appears 10%
Pharro: God of the imperial family and the Kushan clan. Represented splendor and might. Appears 11%.
Shaoreo: Sogdian version of Perso-Median Shahrevar, the god of metal and blacksmiths. Appears 5%.
Atsho: The Iranic god of fire or in Zoroastrian cosmology, the holy fires. In Kushan icons, Atsho is often molded with Hephaestus and Tabiti, the Greek and Scythian deities of fire. Appears 2%.
Oatsho: The personification of the Oxus river. Appears 1%.
Katrikeya: The god of stones and warriors also a Hindu god of war. Appears 2%.
Manaobago: Bactrian rendering of Avestan, good thoughts/Vohu Manah, what this meant to the Kushans, we are not sure. Appears 1%.
Shiva: Hindu god of many things, the eternal dancer. Appears 9%
Ganesha: Hindu god as well, of wealth, wisdom, luck, etc... Appears 1%.
(other gods account the remaining amounts [less than 1%];Ahura Mazda Zeus, Buddha, Nike, Dionysus, Sarapis, Rishti, Yamasho, Teiro [Armenian god of writing], etc)
Percentages of coins depicted during the reign of Huvishka 155-187 CE, prior to the last Emperor of the Kushan Empire. Disclaimer, the Buddha appears less frequently than Hindu or Iranic gods and Buddha is mostly seen in the period of Kanishka I (100-147 CE) and a minority in Vashishka (147-155 CE) Huvishka (155-187 CE) and Vasudeva I (187-230 CE) (Vasudeva I, saw the increase of Hindu deities on coins, before his fall to the Sassanid conquest).
Meanwhile, Ahura Mazda is depicted in coins less than 1%, quite low than what we would expect should Zoroastrianism be a major religion in the Iranic world. However, the dilemma becomes, very rapidly after 230, with the fall of Vasudeva I (187-230 CE) against the Sassanid Empire, we find the approximate changes begin with Zoroastrianism becoming existent and this once obscure god, Ahura Mazda becoming a common coin in the Kushan vassal states of the Sassanid Empire. In otherwords, the Sassanids carried this religion of Zoroastrianism to Ferghana and the regions of the east, but did not carry the polytheistic Iranic gods that were highly revered by the Kushans. I am not too familiar with Arsacid coins, but I expect that the trend is the same or similar and the Kushan and Arsacid had many things in common between each other.
2. On the point of the Fire symbol, fair enough. I retract that statement of mine.
3. You could say the Arsacid were related to the Persians in the same way that the Scythians were, but otherwise, no. The Sassanid did much to promote this understanding also and the arrival of the Dahae invasions of Seleucid Parthia and Mazandran, reminds of this reality.