Robert
In answer to your questions:
I have not had reason to use the mag_align cli command because I use the mag align section which has always worked for me.
'I need to learn how to tell the compass which direction is forward': what you do is mount your gps then configure Inav with the correct compass direction. on the gps/compasses that i have used forward direction is the opposite side of the wiring harness. Using that information has allowed me to quickly setup the compass. If you mount the gps in any other configuration you will need to go into Inav and configure the gps mounting correctly (it can sometimes take several attempts of the different options to get it working. For me, I have learned to take the quad and a laptop outside away from electrical/metal influences in my shop to do any adjustments to the compass settings.
one of the more common installation mistakes occurs when the gps is installed upside down. I won't go into a detailed explanation here, there are ample photos on the internet.
'Two potential gps location are above the camera or on an arm'. It is recommended to keep the gps as far away from electrical components and wiring as possible. I mount my gps on a short 3D printed pedestal mounted on top of the quad or also on a 3D printed mount on the back of the quad which places the gps between the two arms. however some have reported success with mounting the gps on the frame directly above the FC or on an arm (zip tied against the motor wirings). Experiment with the mounting position. If the qps is slow to lock onto sats then try to find a better location. Keep in mind that as you fly, you will/can occassionally lose a satellite lock so the quicker it can reconnect the better.
below are the two mounting options described above. the pedestal is about 45mm tall. On the rear mount photo you can see i mounted the gps with the wiring harness sticking out the back. The reason for that is my gps must be unplugged or the quad will not go into DFU mode.
These mounting locations allow me to get really quick sat locks. I live/fly in a hilly wooded area so getting a quick lock is not always easy.