Imodane, Pyrohammer working incorrectly with cards that happen to do damage to secondary targets. By card rules text and the rules of the English language as long as it “targets only a single creature” it should still do damage to opponent, regardless of secondary, non targeting effects, including damage effects as long as they are not targeting damage effects.
For cards like Fear, Fire, Foes and Hungry Flames should trigger Imodane, Pyrohammer, as they target only a single creature and deals damage to that creature.
Spells like Flames of the Firebrand if targeting one creature should trigger, but if targeting two or more should not.

“Whenever an instant or sorcery spell you control that targets only a single creature deals damage to that creature, Imodane deals that much damage to each opponent.”