Good analysis, but I wonder: the clips in the FRX videos are made for the video. They have no incentive to play an extremely goodgamewith them. Most importantly, I think they create amap in the worldbuilder to control everything, so no gaining Great People nor necessarily building wonders. Moreover, they often use those videos to tease us in the future LUA and CUA. We deduce the babylonian 100% eurekas or the vietnamese "districts on features" from the updates videos. There always had one or more hints about what the abilities are, and I think it might be one. I'm not sure FRX comm department would go through the hassle of playing a complete game until the Renaissance Era just for some extracts, while the worldbuilder tool is way more useful for that.
Moreover (again), additional trade routes for Portugal or Joao III would perfectly maje sense as a UA. We only have 5 (IIRC) civs/leaders that give additional trade routes: the Cree from pottery, England from settling on other continents, Persia from political philosophy, Mali from golden ages and Phoenicia from the government plaza. There is still room from additional trade routes capacity bonuses and, once again, it would perfectly fit with Portugal.
So I think Portugal being able to have more trade routes isn't that far-fetched and could be an extremely reasonable UA.