I’d be happy to respond within the limited confines of this forum:
1) For thousands of years, Jews have argued with Christians against J-s being moshiach. One of the major arguments was that he died without a redemption. The Christians respond that he will come back a second time to complete it. This is precisely the argument that the modern-day Lubavitchers are making regarding the Rebbe, z”l. If you combine this idea with the Rebbe’s statement רב” – הוא זה שמקושר עם עצמות ומהות א”ס ב”ה, and that we do not have to connect to Hashem, but only to the Rebbe, we’re not too far off from the Christian belief.
This is not to say that a Rebbe is not a vehicle to bring a person closer to Hashem, because he is, and this is likely what the Rebbe meant. But the language in the Maamar can be quite dangerous.
2) Even if we do accept that Moshiach can be someone who has died, the argument that there are much greater people among the dead than the Rebbe is very strong. The common Lubavitcher argument is that Moshiach needs to come from the generation of Moshiach. This argument might have worked in 1995, but we’re 25 years later. Many, if not the majority of Lubavitchers today have never seen the Rebbe. In another twenty years, if Moshiach has not come yet, C”V, the situation will be even worse. Will they continue to say that the Rebbe is the gadol of their generation, having died 25/45 years prior? That’s absurd, and it against everything the Ba’al Shem Tov stood for.
And by the way, עקבתא דמשיחא started with the Baal Shem Tov, although there are references prior to him as well. If you’re going to say that we are still in the generation of the Rebbe, then you can very easily say that we are in the generation of the Baal Shem Tov. He was much greater than the Rebbe; as the Beis Avrohom said, if he had lived during the time of the Avos, there would have been four. But people don’t go around saying that the Baal Shem Tov is Moshiach.
3) Gedolim, including gedolei hador, mekuballim throughout the generations, have been wrong about Moshiach’s arrival. Countless people have made calculations and been wrong. The Rebbe said that Moshiach’s arrival is imminent. But for Hashem, who lives outside of time, imminent is a relative concept (a thousands years being a day, for example; of course this as well is only a moshol). If all those gedolim who predicted Moshiach’s arrival were wrong, the Ramban and others, why can we not accept that the Rebbe was wrong as well?