If a merged creature dies and that creature didn't have flying, each card that was part of that merged creature is returned to the battlefield. Each one has a flying counter on it.

If Luminous Broodmoth dies after losing flying but not losing its triggered ability, its ability will trigger for itself.

If Luminous Broodmoth dies at the same time as a creature without flying, its ability triggers for that creature.

Luminous Broodmoth's ability triggers if a token creature without flying dies, but the token won't be returned to the battlefield. If that token was a merged creature, any cards that were part of it will be returned to the battlefield.