Your test is more like the following:
Consider a password consisting of random words each chosen from a 4k dictionary. Say you choose 10 words. Then your password has log_2(4k)*10 entropy.
Now consider a validator that tells you when you gets a word right. Then you can guess one word at a time, and your password strength is log_2(4k*10). Exponentially weaker.
You're constructing the second scenario and pretending it's the first.
Also in your 50% probability scenario, each word is 1 bit, and even 50-100 bits is unguessable. A 1000 word key where each word provides 1 bit would be absurdly strong.
You're still missing the point. The numbers don't matter because it's copyright infringement as long as I can get the book out. As long as I know the key, or the seed, I can get the book out. In court, how would you prove it's not infringement?
Because you put the book in. Again, this is measurable. Compress the book with a model as the predictor. The residual is you having to give it the answer. It's literally you telling it the book.