the witcher 3 is great It shows short sentences but Geralt doesn’t exactly say that and usually develops a little more. If you are impatient, you can choose a simple sentence and skip the talking without losing the general meaning. You can also listen to the whole thing.
those promises don’t constrain them in any way other than PR. They can promise anything they like and are completely free to do whatever they want after that
The game doesn’t really want you to spare enemies. It’s just that there are 3 different ways to play the game and 3 different flavors of the story : low, mid and high chaos. I think you should feel free to massacre everyone, and then maybe start over a new game and try lower chaos !