The rules of nevua are clear. If you say a bad prophecy and it doesn’t’ come true, then it doesn’t mean you’re a false prophet. If you say a good prophecy and it doesn’t come true, you don’t get to qualify it and say “he meant this but now it’s up to us to finish it”. No beis hamikdash today, we’re still in golus, and the rules of golus apply. Even die-hard Lubavitchers will agree that we’re still in galus and that there’s no beis hamikdash, even if they view 770 as the beis hamikdosh in golus.
You’re a false prophet. The Rambam is rather clear on that. The Rebbe predicted multiple times moshiach was coming in the 80’s. So if you call him a navi, he’s a navi sheker.
The answer is given in the book of Jeremiah. Jeremiah had been prophesying national disaster. The people had drifted from their religious vocation, and the result would be defeat and exile. It was a difficult and demoralizing message for people to hear. A false prophet arose, Hananiah son of Azzur, preaching the opposite. Babylon, Israel’s enemy, would soon be defeated. Within two years the crisis would be over. Jeremiah knew that it was not so, and that Hananiah was telling the people what they wanted to hear, not what they needed to hear. He addressed the assembled people:
He said, “Amen! May the Lord do so! May the Lord fulfill the words you have prophesied by bringing the articles of the Lord’s house and all the exiles back to this place from Babylon. Nevertheless, listen to what I have to say in your hearing and in the hearing of all the people: From early times the prophets who preceded you and me have prophesied war, disaster and plague against many countries and great kingdoms. But the prophet who prophesies peace will be recognized as one truly sent by the Lord only if his prediction comes true.”
Jeremiah makes a fundamental distinction between good news and bad. It is easy to prophesy disaster. If the prophecy comes true, then you have spoken the truth. If it does not, then you can say: G‑d relented and forgave. A negative prophecy cannot be refuted – but a positive one can. If the good foreseen comes to pass, then the prophecy is true. If it does not, then you cannot say, ‘G‑d changed His mind’ because G‑d does not retract from a promise He has made of good, or peace, or return.
It is therefore only when the prophet offers a positive vision that he can be tested.