You know what, after taking a look at that cover for EP8 again, it seems to suggest to me that Ange and Beatrice are the ones who are on the outside and everyone inside the picture is 'happy' but also... dead. Well, Lambdadelta and Bernkastel are also there, outside the picture but...
I wonder if it was Beatrice who survived and not Battler? It's possible that she left her last instructions for the servants on the morning of the 4th and then just... left the island? We have all sorts of reds about how the outside can't interfere with Rokkenjima but there isn't really anything to say that people can't leave. Especially if they take the last boat on the island, say maybe in the submarine pen?
EDIT: It could also mean that the people in the painting are fictional and so is the 'Battler' in the story. And perhaps Beatrice/Yasu left the island in search of the real Battler, who really didn't come to the island that day.