Do not use cleaning slime on this kind of keyboard (the brightly coloured putty designed to pick up little crumbs and dust). It gets trapped under the keys and does not come out without a lot of messing. I speak from experience and I post this here to save someone else the same hassle.

Battery reconnection was insanely hard. It's the last time I try this. I'll just give it to a proper watch repairer and go to the pub. If you get a black and white batter on the watch screen with a white x in the middle, battery isn't connected.