If I play Mass Mutiny and target a Heliod and the God loses devotion by coming under my control, would the legendary enchantment stay under my control as it would then no longer be target-able as a creature before the spell fully resolves or would the card just revert back to its original owner as it would no longer be a creature once coming under my control if I lacked the devotion?
You would control Heliod as an enchantment until end of turn. The control effect would wear off and return control to your opponent where if they have the required devotion it would be a creature.
You keep Heliod for the duration specified (until end of turn). The Heliod even untaps and gains haste. It was a legal target at the start of Mutiny's resolution, and that is good enough for the entirety of the process (see CR608.2, 608.2b, and 608.2c).
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