Then no one can dogmatically say that the Sanhedrion may not be reinstituted or the Temple rebuilt or a king anointed because Mashiach hasn't come, correct?
Logically, that is correct.
That seems to be the only logical conclusion. Which makes it even more puzzling why anyone would object to starting the Sanhedrion, rebuilding the Temple, etc., because "Mashiach has not arrived" when he will only prove himself by doing these very things. It is for this reason that I do not understand the absolute passivity of some people. Even if a great Tzaddiq like the Rebbe (zt"l; zy"`a) ruled that the Sanhedrion could not be built fifty years ago because Mashiach had not arrived, how could the non-arrival of Mashiach be invoked perpetually against doing these things? This would prevent Mashiach from doing them, G-d forbid.
Alouette may have hit on a way to explain this in her latest post. Perhaps the Rebbe was saying that a true Sanhedrion cannot be convened in our day because they cannot meet in Lishkat HaGazit, but one day someone will restore Lishkat HaGazit to the Sanhedrion and this person will be Mashiach. Until then, we have the restoration of semikhah and this Beit Din.
The question of the Sanhedrin and Mashiach is also dependent on the recognition of true prophecy, which can only take place when a majority of Jews in the world live in the land of Israel. I believe that if the majority of Jews did indeed live in the land, there would be enthusiasm for establishing a true Sanhedrin.
And there are opinions that, strictly speaking, the Temple isn't needed for these things to happen, but that the finding of the Ark of the Covenant, etc, will suffice.