I slightly disagree about someone gaining no experience from simple tasks (skilled adventurer killing an animal) I am of the opinion that there would always be more to learn. That it would be akin to practice with a dummy, sure you dont necessarily learn new things, but you gain more experience and further ingrain muscle memory.
That being said it would have to be a very small amount of XP awarded (single or double digits at the most)
As for your second point, reduced XP from combat after a non combat solution is reached could be workable. You wouldn't even really have to tell the player that they are getting less XP their knowledge in this scenario shouldn't impact their enjoyment.
Ultimately I agree with your overall point that people will play the game differently and more power to them. Sometimes killing will benefit them, others they will miss out on something and that is fine.
I simply hope that those who want to play a little more on the alternate solution side will get comparable rewards and enjoyment.
