Given that there are only two components that could possibly be the problem and you've pretty much eliminated one of them, whatever remains, however improbable, must be the truth (to mangle a Sherlock Holmes quote).
But yeah, I'm with you in saying your next step should be to replace the button flex cable. The thing is, I think you already knew that but just wanted reassurance. So yes, go for it and let us know how it all turns out!