Eric decided to make himself the villain to prevent the humans and vampires from killing each other because he's technically neither (a hybrid) making him the common enemy and uniting them (which to be honest would be hard to believe).
They faked eric's death and now he lives at his mom's old place all alone (so people believe he's dead). They don't live together but in the end, it shows that Shell often visits Eric (for both business and personal reasons). It's not the happiest ending, but it's bitter-sweet.
In terms of the world-changing, the vampires won't go extinct, but they'll soon have to start moving south as climate change happens.
In terms of the "or beyond that", Shell is saying that the world is a cruel place and that he hopes to be with Eric, not in "the end" where everything is gone to shit, but when things in the world are more "happily ever after."

Idk if anyone will see this. But for some reason I don't understand the end. Shell saying something along the lines of if there was another option they would have met not in the end but beyond that. What's the end ? What's ending ? I'm confused. Is shell gonna die super soon because of the heat or is that still years away. Will they have a life together or nah? I'm so cofnsued. Everytime I read it I'm still confused at the end page.