Huh. It's funny, about open door=) But snail is a small pet. So I think the bigger obstacle for him was the wall of Terarium. I mean, each of us could live in the same situation and not make out that we are closed ourselves in the same "Terrarium".

Yes, Sometimes we miss the details in order to give the viewer some freedom.
Concerning your questions:
Some creature has picked the Snail up(you may see opened eyes) and carried him out of the Terrarium(moving ZZzz). This mysterious creature help Paul to escape =).
Probably, We will show this creature somewhere in other episode in second season. But for now, all information about the creature is a secret.

And Sorry, but maybe, I don't understand you. It seems you looked the cartoon absently. Because, I've shown this cartoon to my students and they have understood what have happen. =)

Anyway, thanks for watching and for your review =)