I think most of the issues you are describing are due to the limitations of the gameboy itself, and I didn't have the same experience playing on original hardware (though it was difficult).
The friction mechanic is to compensate for the movement in an analog d pad usually, so I never noticed it being an issue personally, and again not sure if it was because I was playing on an actual gameboy, but I thought what you could and couldn't jump on seemed obvious (and it would have been very rare for a gameboy platform to give you any instruction, so the experimentation felt a little authentic).
Gameboy does not have the memory capacity to store all AI states, and memory management often involves instantiating the same object at different points in the level when the screen is triggered at a certain point, so the lack of movement when the player is off the screen is actually the norm, and involves some painful memory tricks if you want to do anything different.
Just giving you my opinions around what you said, as somebody who has developed multiple gameboy games, both in GBStudio and in C. Personally I thought some levels were frustratingly hard, and would have benefited from a checkpoint.