It's definitely possible that you're right and major events can't be changed. The only hint we have that some things might be able to be changed is the brief time Krystals name changed if you didn't give that pendant to Khloe. I believe the masked man mentions observing small changes at one point as well, but saying large changes seemed impossible, or very difficult to pull off.
The sides can't change large events but are trying to make small changes that do seem to be possible. Perhaps hoping all the small ripples will combine to change something big, since they can't directly change anything major. Sort of like a rogue wave, where lots of small waves if they sync up just right can form a massive one. The very name of the game itself might be alluding to this, Ripples.
