I've just finished the current demo and just to see if I've done everything possible, here are a few questions:
Is that strange stone in the witch's house still useless?
Can you only get 3 souvenirs?
Is there no way to finish the Medicine quest?
1. If you're referring to the monument outside, then yes it can not be activated in the demo.
2. I'm not entirely sure what exactly you're referring to with this one. If you mean the special items that Red can wake up with then yes, you will wake up with one of the 3 special items depending on the answer you give in chapter 2 to a particular question.
3. The medicine quest can not be fully completed until chapter 7. So in the demo you can only get as far as making it and then finding the man has vanished.