I'm A Banana
Potassium-Rich
What about using it AGAINST creatures vulnerable to fire.
Does this invalidate the whole Endure elements ritual/power/spell whatever since Faerie Fire means you don't have to worry about cold conditions?
Well, since it doesn't deal damage, it doesn't deal damage. Maybe it makes the ice elemental a little uncomfortable.
The rules of Darkfire don't say anything about helping you out with Endurance checks or anything, either. It might make a cold, rainy day a little more pleasant for you, but you can't warm your body on a candle's flame.
I should point out that still, in the scenario, it wouldn't grant an automatic success -- the door wouldn't be melted under Darkfire (though with Fireball, it might). If the fire's not hot enough to deal damage to people, logic would tell you that it's probably not hot enough to burn through ice very quickly (it makes sense, while the "only creatures" restriction doesn't quite). But even in that situation, saying "Yes, you can use Darkfire on the door, and it will very slightly heat the ice. Give it a few hours, and maybe it would melt," is saying "Yes, but...", and making sure that the effect is mild.
If the party wants to sit around after the combat for a few hours and melt the door open with candles (as opposed to making Strength checks or tricking the ballista or whatever), I don't see much of a problem there. If they want to melt every frozen door in the place with a candle's flame, I don't see why I should stop them.
I mean, clearly, there would still be better options. Chipping away at the ice with your sword would be faster. But saying it might not be the best tactic is not quite as invalidating and frustrating as saying, flat out, "you can't do it." It says "You were clever, so here is your reward, but not clever enough, so keep trying!"
With fireball, yeah, I'd probably let it start campfires (well, blow up piles of wood), and it already deals damage to vulnerable creatures, and it doesn't last long enough for an Endure Elements style enhancement (and come to think of it, Darkfire might not either).
*shrug* Even if some rule element does spiral out of the GM's control into unexpectedly broken territory, it's not the end of the world. Talk with the player, ratchet it down, make it an adventure, or pump up the enemies to compensate. It's not like you can't challenge someone who can make a candle's heat at will. You've got a lot of tricks in the DM's toolbox to use, even in that worst-case scenario of an abusive loophole you create.