You are missing a point here. In the La Liga, head-to-head matches are considered first when there is an equal-points situation. It's not score difference. Hence, if the points are equal, it means that Real Madrid are the champions.
Yes, this is true. And what this means for both teams is that there is no distinction between drawing and losing - if Atletico lose it's the same as if they draw, if Real lose it's the same as if they draw. So the potentail outcomes are simple:
If Atletico win, they are champions. Also if they don't win and Real also don't win.
If Real win and Atletico don't win, then Real are champions.
Personally I want Atletico to win, as I'm always a fan of the underdog, and I think they deserve it after leading for so much of the season.