Ok, so I had a nonworking shoulder button which I fixed a week ago. This sunday, I thought I'd go for the slightly squishy B-button. Opening up as before, but this time I had to get to the keymat, which means lifting the actual circuit board. Looks like this: Here, I've removed the keymat and also removed the B and X buttons. The circuit board is still attached by the WiFi antenna and the screen cable along the further edge. Anyways, compare squishy button B on the left with excellent button X on the right: You can plainly see the casting remains on the left. On the right, there is a few weird reflexes, but essentially it is flat inside. A few minutes of careful scraping with a knife inside the B button made it sufficiently flat. Reassembly (careful with that power switch!) and testing - Success! The B button is now as excellent as the other three. Yay!