How is Messages traffic routed?
In answer to question 1, it depends. Some carriers allow WiFi calling on other devices (you can check by going to FaceTime &gt; Preferences &gt; Calls from iPhone checkbox). If WiFi calling is being used, your Mac sends it over the internet to the cell carrier. If it's not, then it's sent through your phone (I'm not sure if it's sent over the LAN or through Apple's servers to the iPhone, but I think it's the first).
Second question: correct, you'd only be able to send iMessages to AppleIDs.
Hope this clarifies your questions.