This would all get a lot simpler if we'd go back to reasonable copyright terms. Say, the original US term of 14 years. Everything older than 14 years, do whatever you want, feed it to AI, all that stuff, its output is yours to do with as you please. Newer? Pay to license it for your training set, or don't have it at all if they don't want to sell (until the 14 years runs out, then do as you will).

This same issue's been a huge problem for human creators. It sucks that an adult film-maker can't directly play with ideas and characters that shaped their taste and vision when they were a child, and that older film-makers can't play with the work of their contemporaries from the early years of their career. We'd surely get overall better films if IP weren't as restricted—pick a super-famous franchise, and someone would have done some really great stuff with it, that currently doesn't exist because of IP law and the copyright holders getting to say what does and doesn't get made and sold. For god's sake, just look at all the fan fiction and fan films that get made, and people can't make money on those! The 14-year term would totally provide enough incentive for a flowering of commercial works based on recently-out-of-copyright content.