My question now is: Is this doable without ASM (I have absolutely no knowledge of that stuff) and which compatibility problems would this have (although I don't really care about compatibility because I don't expect this mod to be used with other mods as it features a complete arena setup including automatic match management and effectively turns MC into a shooter)? If it is not doable without ASM, does someone have enough knowledge of PlayerAPI and RenderPlayerAPI to explain me how to achieve this kind of stuff?

I also need to make it impossible for the player to look straight up and down but rather with a certain angle and if the player's feet are behind him while he is prone, he should only be able to look upward at a very small angle. While the player is prone, his legs need to go in front of him if he moves backwards and behind him if he goes forward. This means that I need to make the player's head be much lower while sneaking (in vanilla MC the head is not really lower than while standing up) and I need to kind of rotate the player hitbox when the player goes prone. In this game you can stand upright, sneak or go prone. I am recreating the FPS Tom Clancy's Rainbow Six Siege as a Minecraft mod.

Minecraft more player models 1.10.2