Mostly on target
You're right that it reduces the duration the exhaust port is open for, and at bdc due to it not completely opening the port you have also effectively made the port smaller.
The squish doesn't really recover top end. The squish will provide a more efficient burn which nets better economy and stronger more responsive power. Thats it in a nutshell. The physics behind head design are way way way beyond my knowledge. RB actually reshapes the band, reduces the height and cc's the bowl to achieve a specified compression ratio. You can get away with simply shaving some material off the head if its not too far out. The bike will still run with squish way up around 2.6mm but it becomes less stable 1.6-1.8mm is probably alright for a trail riding setup. Going tighter you will get snappier power but may require better fuels (more so the tighter you go).