I plan on flying SFO / PVG / HKG.

I arranged to have a long layover while in Shanghai. I want to visit Shanghai and go to Xiamen with a local friend (Via Bullet train or flight).

My question is, if/can I fly to Xiamen with the 144hour visa?
If I can fly XMN, can I fly out from there and go to HKG or do I have to return back to PVG to fly to HKG?

Is this possible using the train?