Dear Avira,
I’m not sure what you are seeing in the meforshim. We know that Nevuah is not what it was in the times of the Bayis Rishon. It is mefurishe pesukim. We do not need the Vilna Goan for that.
I would have to track down all your sources individually.
Seder Olam is clearly coming from the Midrash. Nothing new here.
I saw this piece from Rav Tzaddok inside before. The same way he isn’t saying there is no A’Z anywhere on Earth, he does not mean that there is no ability to attain Nevuah. It means that the world doesn’t work that way anymore. The Idolatry Age was parallel with the availability of Nevuah. I don’t remember if I saw the Goan firsthand, but I always understood it like this Rav Tzadok.
More importantly, nobody ever says that any prophet who comes along today, will be killed because he is automatically a false prophet.
I intend to look up the Michtav Meliyahu when I get a chance.