
Chapter 61, page 20/21.
We see Shell returning to the south, to the white flower field, and there's a man on a chair in front of the sea. Even thought we never see his face it's obviously Eric. He's even got the scar in the left side of his face.
His death's just an act to solve the situation, Shell hasn't really killed Eric. He uses a metal panel under his shirt as defense, the other is aware.
As for the house afaik there's only the basement flooded, and probably a bit the ground floor. A wooden house isn't unrepairable.

He is there in the last pannel

This is why for some reason the ending always left me with a bittersweet taste and an underlying sense of anxiety. It's just the way Eric was shown almost felt like it was supposed to be ambiguous. I would've really preferred if his smiling face was shown just as how Shell's beautifully drawn smiling face was shown.

Eric took the responsability of what happened on himself. The world was in caos, the true culprit was already dead but that wasn't enough.
If the culprit were to be a human, the vampires would've killed all the humans probably; if the culprit were to be a vampire, the human now know that they're not invincible. Either way, a new war could start after this "incident", with losses for every side.
Eric is both human and vampire, he doesn't belong in either faction. Being him an unknown "monster" with no ties to any race, if he's considered the culprit and is publicly taken care of, then a war will not start.
Why was Shell choosen to be the shooter?
Because he's an Overlord, and technically the head of the central territory, for his father's will. There's not much to say, as the highest figure it's best for him to be the one killing the "culprit", both to settle down the matter and because he knows Eric's plan to fake his death.
Are they together now? Yeah, I think so.
In secret probably, because Eric's supposed to be dead, but some should know the truth like Shell's friend. Probably those who were close to him know that he's alive and well.

Something like that. Since Eric is already free from all the shit in the world, it's most likely that he had the free time to rebuild the house, which was very important to his parents.
(Also, I don't think it fell down in the first place? I mean it was old, for sure, but it didn't fell down? He and Shell had sex there and Eric left Shell in the house and that was it, right? I'm not really sure, it's been so long since I've read this.)

Oh! The chapter they were standing outside the house Eric said it is flooded with water. And then when he said goodbye to the skeleton of his mother it fell down in the background. Eric and Shell looked at it, and shell said "it was as if it was just waiting for you" or something. Hold on lemme find the chapter quick!
Did Eric die?? Tf?! I didn't see him again ;_;