Snowpiercer is adapted from a French graphic novel written in 1982, so the copying may be the other way round (I've not seen/read any of them so can't comment persoanlly).

I found the first half of Wool to be very good and the rest of it readable, although just barely so. Howey is a terrible writer with interesting plot ideas. Howey has a compulsion to explicitly tell you in minute details through every other passage exactly what is on everyone's mind. The concept of using symbolism and actions to portray people seems lost to him. His sentence structure is terrible. He's saved by Wool's very interesting idea and the first chapter of Wool was admittedly very good. I read the entire series and I really think at a high level his ideas a very interesting, but he just doesn't really have the skill required to write well. I am not sure an editor would help, maybe a co-author.

I think you have it backwards. The ideas of Wool are so common in prior science fiction that authors would take them for granted, play with them. The plot is pretty much that of The Penultimate Truth. Howey succeeded because, for whatever reason, people liked his writing or he details of his version more than previous iterations of the idea.

Very possible. Not suggesting that Wool was entirely unique, but more specifically, that Snowpiercer seemed to have taken the entire idea of Wool and just shifted the axis from vertical to horizontal.

There were a LOT of copied ideas. Perhaps none of them were original to Wool, but Wool was not (to my knowledge) as much a clone of any individual work as it was the compilation of a lot of tropes.