Copyright isn't exclusive, so the original rights holder you derived from would still have claim. You only have copyright on the derived work over the creative input and changes you provided, if any. If two people independently make the same creative changes, they both have copyrights on distinct works that happen to be identical. There's a well-known blog post on this subject called "What colour are your bits" (https://ansuz.sooke.bc.ca/entry/23) that goes into more detail, but the gist is that the copyright is not solely determined by a work's representation in the world, but rather by the process to create it (of which the representation is simply a product). If this needs to be adjucated, it's done by the courts.

This is true, but I believe exceedingly uncommon. There's a requirement for a creative element, and if there's only one way to adapt the music to a modern piano it shouldn't fulfill that requirement and be eligible for copyright at all. I would be unsurprised if copyrights like that existed, though.

If it were at all common, copyright would cease to function. E.g. someone could, in theory, write Romeo and Juliet word-for-word from their own imagination and then apply for a copyright. How on earth would enforcement work? There's no distinction between the copyright-free Shakespeare version and the newly copyrighted version, so do performers get to choose which version they're acting under?

Music has, through various court decisions through the years, gained a particularly arcane and arbitrary set of rules on what parts of a piece are copyrightable, and to what extent. It's a mess, as some parts are overly-protected, like the layout of the music on the page (unlike the typesetting of a book, or the font used!), and some under-protected in comparison, like the rhythm of a song compared to it's melody (which has a particularly arbitrary rule of 7 notes long). Though, to answer the question of the last part, if that version was copyrightable, it depends what book the performers are using! Though again, it could be that the essence of the performance is not considered to be a violation, even if photocopying the book would be, depending on what form any differences take.

If you're concluding copyright is impossibly complicated, vague, and confusing and the only reason the system works at all is because hashing out the details in court is either a nuclear option between two large companies, or enforcement on a large scale against small actors is basically impossible (but will unpredictably come down like a ton of bricks on some unlucky individuals), then you'd be right!