At least for the very first part of your question, Nate himself has answered that question in the past:

https://projects.fivethirtyeight.com/checking-our-work/

(Note, he no longer is affiliated with fivethirtyeight, but at the time this article was published he was, and he retains the rights to the politics models that are being tested, and potentially some of the sports models as well, although I'm less sure of that)