You can’t safely use any song lyrics without written permission to do so. Fair use is indeed fuzzy and applied case-by-case, so there’s no empirical measurement regarding how many lines, if any, would be okay to use without that permission. I’d like to think creative people would allow other creative people to use snippets in the manner you describe, but there’s an argument to be made for compensation for intellectual property. That argument can lead straight to court. Will that risk affect publisher interest in your project? It could. I’ve seen publishers weigh projects that involved estates known to be litigious, and it wasn’t something anyone relished. But here’s the thing: a publisher will tell you if your use makes them skittish. You could submit without the permission and, if they want the project, they’ll discuss the copyright issue with you. They’ll probably require copyright clearance, writing it into your contract. Your safest bet is to seek permission. Probably not the answer you wanted, but it’s the path that will keep you safe.