What would happen to the Liga Federal (the predecessor of modern Uruguay) if the Portuguese in Brazil didn't invade? Could the UPSA take over all of Uruguay, or would something else happen?
Well, the Liga Federal at first didn't want to make Uruguay a separated nation, but rather was a political group involving some regions of the former Viceroyalty of Rio de la Plata that were against the elite of Buenos Aires, who wanted a more centralized country. After all, not only Banda Oriental, but also other provinces belonged to the Liga Federal. If there is no Portuguese invasion probably there would be some kind of civil war between the Liga and the supporters of Buenos Aires, the Unitarians.
However, considering that even after the occupation of Uruguay by the Portuguese and the destruction of Artigas' army the Federalists still managed to defeat the Unitarians at Cepeda (1820), I believe that without the invasion Artigas could have defeated the supporters of Buenos Aires earlier, and the would be the strong man of United Provinces, now in a more federalized model. IOTL the Federalists made some concessions to Buenos Aires after Cepeda, but given that Artigas was contrary to it I think now the peace would be made as he wished.