Dear Youdont,
““4) Rav Moshe’s own opinion is clear from his first teshuvah.”
You are discombobulated. The teshuvah prior is regarding Manhattan (you cannot be referring to 1:138, since that has nothing to do with reshus harabbim). Rav Moshe needed to formulate, at this time, his opinion, since Manhattan’s metzious was unlike Brooklyn.”
And,
““Rav Moshe has no intention of counting the city as was evident in his first teshuva. But if one would count and have 600,000 in the streets than Rav Moshe would have forbade the eruv. And it would be null according to his opinion.”
No. Rav Moshe did not need to count the numbers, because his first teshuvah was regarding Manhattan, where he realized that it was encompassed by mechitzos, and so the numbers are irrelevant. Only regarding Brooklyn did he need to calculate.”
I don’t understand how you reconcile Rav Moshe writing to the Rabbanim in Flatbush (1978) that it is all printed at length in every detail in the first volume of Iggres Moshe, with your opinion that Rav Moshe only came up with the three million number in this teshuva and then reworked a year later?
And all those who spoke in learning with Rav Moshe, testify that even to his last days he said his shita is like it is published in chelek aleph.