You know? I wondered about this too. First time I heard of MMX I though 'Mega Man 10?? There are ten games in the series now?' Of course that wasn't the case at all since Mega Man 4 was still recent. Only later-on did I come to know as X being the name of the new protagonist and not the number of games.
Just double-checked and no, the X has nothing to do with "ten". Mega Man X is actually the eleventh release title, with Mega Man 1-6 for NES and Mega Man I-IV on Game Boy all releasing before Mega Man X hit the shelves (Mega Man 6 actually releasing a mere month before X's release).
I think the in-canon reason (at least in Maverick Hunter X) was that X was meant to indicated "limitless potential", and that he was named after the variable x.
Yeah, Dr. Light states this during the Day of Sigma short film, but like you said, I don't actually know if that's what they intended the meaning to be back when the game first released.
"Akumajo Densetsu", CVIII, would be "Legend of the Demonic Castle".
Going off that, I think the reason they chose the whole "Legend of the Demon Castle" title was that, at the time, Castlevania III was the earliest CV game chronologically. So I think it was fitting to refer to it as a "legend", since by the time of Simon, Trevor's story would actually indeed be a legend, so to speak.