Opsec.

He was trying to throw people off his scent.

Satoshi was a pseudonym -- this person does not exist under this name. He used disposable emails. He paid for the domain with an anonymous registration agent. Presumably he only connected with tor.
He didn't want to be found, and he went to great lengths to ensure that.

So, 'z' versus 's' was a way to throw people off his scent.

The way to analyse this would be to graph is 's' versus 'z' over time. Did he use 'z' for the first half of his posts, or the last half? Are they consistently mixed at all times?
And no, I don't believe that Satoshi was a team of people. Single, clever individuals do great things all the time by finding combinations of old ideas into new.
Very few people in the world could keep a secret like this for so long if they were a team.