Now, regarding the statement you linked to, I am glad it is in English
Although I do not have hilchot melachim in front of me.
I am fairly sure that nowhere did it say that moshiach should be a prophet.
Secondly. If the Lubavitcher Rebbe had ruach hakodesh, as you say. Then that does not make him a prophet.
The RAMBAM himself, in "The Guide" puts Prophecy - a Navi - at Level 3.
It occurs in Visions and dreams.
Ruach HaKodesh is still available. And many rabbis are said to have had it. That does not make the said rabbi a Navi.
Even King Solomon was not a Navi , though he wrote with Ruach HaKodesh.
The RAMBAM, in "The Guide" says that Shir HaShirim(Song of Songs by King Solomon) is in Writings and not Prophets, because it was written with ruach hakodesh, not prophecy.
Ruach HaKodesh, and Navi level prophecy, are very different things.
A person with ruach hakodesh, is not a prophet.
So even he had it he is not a prophet.
And being a Prophet, to my recollection, is not a condition of the Presumed Messiah, or the actual messiah.
And if you want to talk about people that "know the RAMBAM better than me", then I can humbly add a thousand rabbis for every one you mention, and each of the thousand believes that the L Rebbe is not moshiach. So this intellectually stifling, anti intellectual argument of yours doesn't work anyway.
And you have to think for yourself and not say "oh, he knows better than me and he says it is this way, so it must be". Because as shown, it doesn't work.
Furthermore, people with good knowledge can be very illogical..
As an example. I heard that the Steipler Gaon said of ovadia yosef, that his sources , his breadth of knowledge, is fantastic, his responsa are interesting for his sources, but his conclusions are totally wrong.