The opening is basically singing about dedicating your hearts ("Shinzou wo Sasageyo") so the red spots you're seeing are the hearts. We saw the hearts of the characters earlier and then later the hearts of the animals. As for the "beasts" running together over the land, I always saw that as a predator vs prey kind of thing.
Poor Gelgar, the guy just wanted a drink and he didn't even get that much. And poor Nanaba...She went back to a really dark place in her last dying moments there...
Ymir with the reveal and (literal) cliffhanger!!
He's a titan, she's a titan, you're a titan! Everyone's a titan!! :D