“Rav Moshe says that the population would be estimated to produce the amount on the streets, and it is likely that different cities vary on this calculation.”
No, Rav Moshe at the end of the day did give a number. You do not realize that you are not making any sense. Rav Moshe’s final number is three million people. If the population of a large city is any less, then one may think that it’s a reshus harabbim, and that is why they should not establish an eruv. According to you, Rav Moshe should have argued, when he was made aware that Brooklyn and Detroit has less than three million (and the reason not to make an eruv was because one may think that it was a reshus harabbim), that it is irrelevant. Since, “different cities vary on this calculation.”
“Also, the area would be 12 mil square [slightly over 7 miles] (less than 9 miles) like it was in the dessert encampment. So then if one would measure in such an area enough people to have 600,000 in the streets then it would be a reshus harrabim for sure. [And Rav Moshe would have told the Rabbonim that there is no way to make an eruv without dalsos and so on.] But if there is a street anywhere in this vicinity that carries 600,000 by itself, than that street alone would be a reshus harrabim for sure.”
No. It is slightly over eight miles. Go learn Rav Moshe’s shiur amah, in regard to hilchos Shabbos. Since it is impossible to know how many people are actually in the streets at one time, Rav Moshe resorted to giving a number, three million, the amount of people in the midbar. You are avoiding this fact because it makes you uneasy, and it would allow an eruv in Brooklyn, if the metziuos is otherwise.