"The ability for a site to opt out, and have their data deleted from the AI's training data within a set, but short, timeframe (say 48 hours) must be included in this,"
It wouldn't work. Once they start training their model, the content is baked in there if the software chooses to retain it. You can't strip it out afterward. Nor am I comfortable making this opt out. Here is my suggestion.
When you want to train an AI model, you get an explicit license for everything you throw in. If you want this book, you find out who owns the rights to that book and ask them for a license to train your AI on it. If it's in the public domain, you're good and can use it. If it's under a license that permits you to use it for your commercial purposes, you're also good. If they give it to you for free, great for you. If they want money, negotiate with them for how much. If you find it's too expensive to negotiate with individual authors for individual books, feel free to try to negotiate with a group of them en masse. Some authors might not agree to that. Too bad for you, you can't use those books until those authors die and the copyright period after death has lapsed, or you can always come back and try to negotiate some more. Find someone else's book. Replace book with site, song, or any other thing that can be copyrighted.