Good points from the OP.
It is however missing one of the largest changes, which it that you get Advantage from High Ground, and Diadvantage from low ground. Advantage/Disadvantage is an effective +/- 4 to dice rolls, which means a +/- 4 to AC.
Remember the Harpy fight, where you're stuck with permanent Low Ground Disadvantage, with difficult terrain and climbing between you and the closest high ground?
There are a few encounters they have to balance better. This is one of them. It is almost unbeatable if you don't know what is coming or have a pretty high level. However in this special case I noticed that the song varied in volume depending on where my camera was so I assumed where it is highest is where the enemies are hiding. So I moved my party to a far better starting position without knowing and initiated the battle by being too close to them rather than talking to that boy (which I did in my second playthrough).
