Top case wouldn't need repaired. The keyboard and power button are part of the same assembly. So to replace the power button, you would replace the entire keyboard. I did this repair on a 2010 MBP that kept turning off during the boot up sequence. The person I bought it from figured it was a bad logic board, but I took the gamble, and sure enough, it was the keyboard causing a short or whatever the problem was. New keyboard made it run like new. I noticed a lot of sticky nastiness on the old keyboard after I removed it, so I'm sure that was also caused by liquid damage. The logic board was fine. I think this problem is often overlooked when people report water damage. People are always quick to say logic board, but the keyboard really traps most of the spill it looks like (of course it depends on the part of the device the spill occurred, and just how much liquid got in.
The problem of the MBP shutting down during boot was due to a faulty keyboard. I replaced the keyboard with a new one, and now it runs like new. Someone had previously spilled some liquid on it, because it was sticky and nasty when I ripped out the old KB. I'm guessing there was a short somewhere.