Five keys have stopped responding—must I replace the top case?

Hi folks!

Five keys (F4, 4, R, F, and ]) on my mid-2012 15" Macbook Pro have recently stopped responding. "E" has been giving me trouble on and off, but currently it's working.

Is there anything to do but replace the entire top case?

They went out slowly, at first, and then all at once: the "E" started responding intermittently in November. In December, sometimes my "F" wouldn't respond.

Last week, I removed and cleaned under "F," "R," and "E," blowing out dust and gently scraping around the little rubber pad under the keys. They all worked for a couple of days. Then this afternoon, in the middle of a paragraph, F4, 4, R, F, and ] (mostly in a column, except for the odd man right bracket) all suddenly gave up the ghost.

Now, when I take the keycaps off, pressing on the little rubber pad doesn't register in Keyboard Viewer. I've tried blowing them out again, but no cigar.

For what it's worth, I've also tried a restart, flashing the PRAM, removing and replugging all my USB/Thunderbolt devices, and Mouse Keys is NOT enabled.

So again: Am I doomed to an external keyboard or a $400 top case replacement? Or is there any other option?