My current save is at 16/20, so definitely it's possible to get past 10, but been such a long time that I don't remember how.Did the trade deal and stood at 10
Very important however, is to get to 10 via stealing apples the moment it's unlocked, and before you do any of the post-Festival of Greed optional quests.
First thing you should do when you get access to the mental changes room is to unlock Values room and start corrupting the orb that unlocks stealing.