I’ve the same problem (many years later). The keyboard ánd trackpad stopped working at the same time. Nothing seems to work (like PRAM and SMC reset). External mouse and keyboard works fine though. I’ll try to replace the trackpad flex cable. And I hope it will do the trick. Apple says I should replace the top case. But I’m not sure why that would solve the problem. Edit: I replaced the trackpad (with the original cable), that worked! Both keyboard and trackpad work fine now!