I have a feeling some of your issues are cause by your Mac. Do you have access to a PC running win 7/8/10? Make 10000% sure your grbl sheild is seated fully into the Arduino. Take the Arduino out of its metal case and connect everything together. Make sure you haven’t accidentally shorted anything when soldering.
Make sure that you are not running Easel or UGS when you try to open the port. Only one program can access the port at a time.
Turn on soft limits ($20=1), when you can talk to the serial port (you can do this in UGS). That will stop the runaway crashes (must home the machine for this to work right).