The mouths of sea stars are located on their back side, therefore it is only logical to conclude that a Pokemon based on a sea star would exhibit the same concept. \m/
As for Magnemite/Magneton, they probably generate a strong-enough current that allows them to absorb electricity using their magnets. As for how they eat Pokeblocks, I would envision that they use some conversion mechanism on their magnets that would allow them to "digest" the Pokeblock and perceive it as having some sort of taste.