I see.The key is to gain 550/1000 before Night6 complete.And then u can unlock magic shop and buy that damn necklace.
So the trick hides in the order u complete the research tree.
Gain 250 in A.I before night 5.(don't know if it's necessary but it reveal something about why Bonfie is upgraded in night 6)
Gain 550 in paranormal before Night 6 ends.
Gain 800 in missing person
And security is necessary but we don't know how many points it need.
You are a genius mate!