What if they swallowed one of those super-powerful rare-earth magnets? Then they could do the trick (and possibly squish their innards together, which could be a problem), and then just poop the magnet out later.
Also, notice how the heads of the hammer are placed on the rounded parts of the chest and abdomen, so in a sense they aren’t clinging to a 90º vertical surface, maybe more like a 50º gradient (on top of whatever they gain by leaning back ever-so-slightly). Plus like Mangetout said, the clammy skin provides the proper friction, and I also think the long handles might provide some form of balance.
Yes. The adhesion of the smooth metal hammer head on the clammy skin means that any downward motion tends to rotate the hammer in such a way as to press the handle more firmly against the skin, further improving grip.