maybe the RAMBAM was only listing what would -certainly- happen. And the reason why, is so that we do not identify a false one.
furthermore, you mention he lists what he will certainly not do. No doubt he was aware of the tradition of moshiach ben yosef, and he did not list that in his certainly do nots for moshiach.
I agree with you that Tzvi`s link is rubbish. It is out there.. Tzvi, you posted one site where somebody nobody had ever heard of thought he had gone up and spoken to moshiach.
I saw a similar site(called erevmoshiach) about children that are meant to have seen the time of moshiach, they tried to predict the near future and got it completely wrong. The site was pulled down soon after, because - to paraphrase - "it gave away too many secrets" and "kabbalists" objected to the site being up.
The site was taken very seriously, the child would speak, and then scholars would analyse his words and find these obscure sources to backup lots of what he was saying.. and they would say he knew things that he could not possibly have learnt himself. And the use of the method was justified, by a quote from the zohar that children would see the prophecies of the future.. It had a forum too, and many people there did not agree with the stuff on the site, but were very knowledgeable.. One even mentioned a kabbalistic source for the magen dovid, but I did not note it down.
That site certainly took more care to be accurate than the site or article posted to start this thread!