For what its worth. I corresponded a while ago (through the official chabad website) with regards to the issue of the Rebbe being moshiach and the proliferation of this ideology within chabad. The responses I received were as follows:
No, the official Chabad officials do not hold that the Rebbe is Moshiach.
There is no underground movement which unoffficially permits for statements to the contrary to be made. I'm not sure why there is a proliferation of such beliefs in some parts of Israel. In North and South America, Russia and Europe that is not the case be that as it may, it is individuals not the official stance of the movement.
The Rebbe not only did not want anyone to perpetuate the idea that he
is Moshiach openly as you said, but he didn't want the idea to be
perpetuated at all, period.
Along with the other mitzvahs and mitzvah campaigns which he initiated
the Rebbe encouraged us to spread the word that Moshiach is coming, he
did not say 'tell the world I'm am Moshiach'…
And another related response from Rabbi Menahem Posner:
a. I do not believe that the Rebbe ever said he would be Moshiach
after his death. Rather, the belief seemed to evolve as follows: More
than anything in the world, the Rebbe wanted Moshiach to come. He
constantly spoke about his arrival and worked tirelessly to bring
Jewry to a state where they would merit his arrival.
It was not a huge leap for some people to connect the dots and assume
that the Rebbe himself would one day be Moshiach.
After his passing in 1994, people were at loss regarding how to
continue. They just were not ready to say goodbye to the Rebbe and to
his vision of Moshiach’s imminent arrival (and rightfully so).
Different people reacted in different ways.
Many Chassidim saw their mission to continue the Rebbe’s work of
spreading Judaism wherever possible as their response to the loss
which we and world Judaism had suffered. Others continued to refer to
him as Moshiach as a way of expressing their conviction that he and
his message live on.
This well-intentioned, but misguided, response is the root of what you
sometimes see today.
Some of the Rebbe's works were edited and others were not. I am not
aware of him editing and approving a work with "melech hamoshiach"
appended to his name. My assumption is that this was added afterward
by Mishichist elements.
The Rebbe did indeed speak about prophecy in our time prior to the
advent of Moshiach in the sicha that you reference...