Upon entering Brazil, the Argentinian players shown their passports to Brazilian authorities, which means they could see where each player had been in the past 15 days. They let them in.
Sorry I somehow missed this post from a while ago. You are correct in every assessment you made, except this one quoted here. 2 of the 4 Argentinian players in question have dual citizenship, and hence, a second passport. They could've easily entered the country by showing whichever passport didn't have a "British stamp", but I find it weird that the other 2 players (Martinez and Romero) only have one passport... Anyways, I can cite me as an example: I have both Brazilian and Italian citizenship. I once entered London with my Italian passport, but then showed my Brazilian one to get out, and got an ear full while leaving (from British customs), but not when arriving in Brazil, so besides all things being fishy (and I completely agree), I can attest to the fact that Brazilian customs authorities are severely incompetent.
I also agree with you that both the Brazilian players and manager (Tite) wanted the game to be played, but the Brazilian confederation (CBF), being the corrupt organization that it is, might have worked behind the scenes for all of this "circus" to happen as some kind of retaliation since they were not able to call up the Brazilian players from the Premier League (most notably Alisson, Fabinho, Gabriel Jesus and Firmino).
If CBF's "foul play" can be proved, I really hope that they get sanctioned and that Brazil lose the game (And I have absolutely no problems with that, as I am amongst the millions of Brazilians that rooted for Argentina in the Copa America final, mostly because of the "Mussolini-cosplay-posing-as-president", whom accepted to host the competition, just to keep his ongoing necropolitics agenda... I also personally can't stand Neymar, or any other Brazilian player that support the genocidal maniac), but since it was the Argentinian players that have abandoned the match (and understandably so, since they were standing up for their 4 teammates in question), I fear it's AFA instead that will be punished, unfortunately.