Can I force people in to first person?
Probably not. In vanilla Minecraft, the perspective is only ever changed when the key is pressed (at least in 1.16.3, but it probably hasn't changed).
It is possible with mods, but that's a bit too much effort - everyone has to install the mod (which has not yet been created).
Edit: it is possible in 1.8, just suffocate a player! As soon as a player is inside a block, they will be forced into first-person perspective. So, the answer is yes, if you are willing to kill all your players.