The collective wisdom of the Internet comes to the rescue.
One suggestion stood out – not only because of it’s simplicity but because I have used it many times myself over the years. This procedure requires a very special tool, one that is nearly obsolete in this day and age. I wasn’t sure I had such a rare device.
I did find one, eventually, in the back of a drawer
The mighty pencil eraser, handily attached to it’s applicator wand.
A few quick pokes on the offending contacts, and the resistance dropped to below 1k in most cases. In an hour so, all of the key switches had been reassembled and resoldered.
I did look at the contacts under the microscope but didn’t see anything significant. Perhaps the gold looked a little brighter, but that’s hard to tell on an $80 microscope with 1080p screen that is three inches across 🙂
We’ll see how long the fix lasts, but for now, I am declaring victory.