I suspect that Hellfire is a property of Lucifer, a corruption of soulfire, and that if you use it, you get it as a boon from him. By invitation only.I will go one step further and suggest that Namshiel couldn't use all the power actually available to the Fallen, if the Fallen were released from the coin. The power of the fallen resembles the mantles, in that the more you use it the deeper into it you go. The Fallen are most powerful in their alternate forms as the host gives up more of their identity. This plays into how Jim Butcher treats the Winter Mantle. In the battle on Demonreach Harry is fighting the tendency to draw too much power from the mantle at the expense of his control of it. Butters is only half right about the mantle.
Possibly at any rate, I am sure Lucifer was the first to use Hellfire at any rate. Whether another angel could have done that/could access it without Lucifer remains to be seen - I think it is more of a how you see the world type thing.
I think you are essentially right, from both a logical and doylist perspective. It makes the most sense for Butters to be only partially right. But I don't fully trust Jim any more. I think he has changed his view (in his writing) about how powerful his devices are - so I am not totally on board sadly. Bit of a cynic, me.
To be honest, I just took these to be delusion. Hellfire made Harry's magic hit harder, and the Winter Mantle really DOES make Harry stronger.
Butters just doesn't really believe it, and his denigration of the Winter Mantle helps Harry feel more like a dummy being abused by Mab and his own bad choices, which is all but a hobby of his. So Harry tries to believe it too, and as a result thinks Hannah is also duped.
I mean, Harry ran on super-slippery ice, and jumped, and whatnot. He did things that are normally physically impossible... and all without digging very deep into his Mantle, after about 1 year of practice. Imagine a Knight none too worried about his soul pushing the Mantle to its limits for a decade.
Look, I would like that. Certainly, the exposition by Butters and Harry doesn't cover everything that mantles/sources of power have been able to achieve. So I do wonder about that. Although it just as easily could be Jim didn't feel the need/forgot to tie up every loose thread with his new explanation. I am very sure if someone were to ask Jim did Harry's spells hit harder because of Hellfire, he could explain it in such a way that is was clear that it was more about making Harry more angry than anything else. You have to remember Harry has achieved incredible stuff magically when his emotions are heightened, and there does not seem to be much of a qualitative difference (apart from which emotion he is tapping into) between when he draws from Hellfire or Winter than when he just draws from his fear and anger (like the scene where he killed all those Red Court vampires). Especially when Soulfire spells are clearly very different to any magic he can simply do under pressure - they make his spells more "pure".
But yes totally agree it doesn't cover everything. Butter's hasn't seen much of the crazy strength stuff - but then again, he would hardly need to. The explanation for Harry lifting 800lbs is mostly that having a crap load of adrenaline in his system and without the natural limits of pain, exhaustion and fear the human body is quite capable of pushing past it's normal limits. The running better on ice could be less physical and more mental than it appears (for instance, if it made his decision making and logic predicting parts of his brain run faster, more efficiently with less error he could naturally achieve that - half the reason we fall and run into things is our brains predictive functions are not accurate or quick enough).
Fcrate - that was the idea originally. Unfortunately we have seen very little evidence of that. I disliked the explanation too - but I think Jim went to the effort of writing that scene, so he was trying to get a point across to us. He could return to his original idea any time but we will just have to wait and see.
Snark Knight - Interesting thoughts. I think so too, it plays into the whole selfish vibe that the villains of the series have. My theory is the Fallen are unable to use Soulfire per se, as they are no longer connected to Heaven, and also perhaps because they have no desire to create anything original. Only destoy.
