Regarding your first point, A theorem from statistical mechanics states that we should expect the first signs of forced change to appear in the free modes of variability of a system. So change might look like natural variability that gets a little large or hangs around longer than normal. I don't know much about the Medieval Climate Anomaly, so don't feel comfortable trying to answer your question. The main El Niño mode is clearly the big dog, but the NPM and other structures also play a big role at times.