I have a friend named Michelangelo. His friends call him with the whole name, Michelangelo, even if it's a mouthful.
His aunt is the only one that calls him Angelo, but several people call him Michèle or Michè. His fiancée calls him Mìchi (or maybe it's Miki -- they're pronounced the same).
The whole family has lived in Florence for as long as anyone remembers.
As for the alternate spellings, in Italian the name was once either spelled Michelangiolo or Michelagnolo. Both angiolo and agnolo are archaic forms of angelo (angel), the latter conflated with agnello (lamb); in the Decameron (around 1350 CE) there's a novel referring to the Agnolo Gabriello (the Archangel Gabriel), and to this day some old stone street plaques near Piazzale Michelangelo in Florence (around 1850 CE) show the spelling Michelangiolo, which is also to be found in many books from those years.