The Uruguay war of independence was weird. Yes, the Portuguese invaded what was La Banda oriental back then in the early 19th century and it was annexed for a few years. Brazil gained their independence right in the middle, I think. So yes, you are correct, the official declaration of independence in 1825 was from Brazil, by a group of "patriots" who supposedly liberated the Banda Oriental. The language never changed, that i'm aware of, though. In reality, in 1825 Uruguay was not really an independent country because Uruguay became a province in the Rio de La Plata region. I think it was only in 1830 that England interceded and we ended up becoming an independent country. The history of such a tiny regionis so convoluted, it gives me a headache :-)
I always think of the first "independence" as the Artigas revolution that created La Banda Oriental, and that war was against Spain. I think that because they teach us that Artigas is our national hero, Uruguayans tend to think of that first revolution as to what ultimately led to our independence.
No, Uruguay never belonged to Brazil, and the language has always been Spanish. Uruguay's independence was from Spain.
People near the Brazilian border do speak both languages though. Quite a few Uruguayans speak a little Portuguese, because of the proximity to Brazil, the Mercosur, and also lots of tourism going back and forth.
How do you like living in SP? I have a childhood friend living in Rio and she loves it, but SP and rio are very different.