, Your box definitely is different from mine. Where you have your AV port, it is always SPDIF output on mine.
You're right, the RESET switch is inside the AV port where you can feel the click when press it with a toothpick. For me, I can't reach the RESET switch thru the tiny hole on the bottom with a toothpick.
If your box is running Android 5.1 then it must be S905. All boxes I knew that equipped with S905X are running Android 6.0, although there's not much difference you will notice. The reason, I believe, is because of Amlogic.
It's no much difference if you use device tree of 100M instead of 1Gbit, only the ethernet doesn't work. It can still boot.
Perhaps you knew about boot issues of recent 7.90 or 188.8.131.52 builds. In may case, it stays at the box boot logo. If it is a fresh install, it only fails to boot the second time, i.e. after the ext4 partition was extended.