I would reset the whole iPhone and start over. Most likely the file was not read correctly after the transfer from the Android phone and the message history was corrupt/moved.
Look at this tutorial how to transfer messages from Andriod to iPhone.
Here is the official document from apple about "Move from Android to iPhone, iPad, or iPod touch".
In answer to question 1, it depends. Some carriers allow WiFi calling on other devices (you can check by going to FaceTime > Preferences > 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 ...