1. I've never seen that phrase before. It might be something you can't understand until you read the manga. One possible interpretation (not having read the story) is that the girl builds a mental shell to protect her from the pain of abuse. The shell could be associated with a playground swing, somehow. Perhaps she goes to a playground to get away from it all. That's my best guess. 2. This is a weird metaphor. Again, I'll give one possible interpretation. A life that is hanging precariously against (over?) the sea could evoke the image of someone hanging from a cliff and barely holding on with their hands. One more push will sent her over the edge. The "transparent sea" may refer to the issues that the author does not clearly present, but only hints at. These might be wrong, but I hope this helps! These questions are very subjective, so many people will have different answers, I expect.

I searched for 'swing shell' and discovered that it's a women's garment - a loose-fitting sleeveless summer top. I don't know if that has anything to do with your comic, though.

"Swing Shell" is not a phrase in English. We can only guess the meaning by reading the comic. If someone's life "hangs precariously on something", it means that it is very delicate and fragile - her life could fall apart very easily if some small thing goes wrong. You can imagine hanging your coat on a very loose hook which could collapse at any time. The second sentence does not otherwise make sense.