A few assumptions/thoughts on the build:
1. You're attempting to build a "medial" template, on par with ghouls, skin changers, and second-sight / SL Proximi. Most
minor and medial templates are immune / resilient to the Masquerade/Veil/Mysteries/Mask of their parent supernatural.
Considering the large part that Disquiet plays in Promethean, I recommend a +2 bonus resistance to Disquiet, as well as an
Unseen Sense (Pyros-based creatures), over the blanket immunity.
I'm not sold on the automatic Unseen Sense, and I wasn't inclined to grant immunity to Disquiet; but resistance to it, both in the form of bonus dice to oppose its onset and perhaps a faster recovery time, would indeed be appropriate.
2. The Hephaesteans themselves don't produce Disquiet or Wastelands. However, they still risk creating Firestorms on
any attempts to use the Divine Fire recklessly; and disquiet or wasteland effect produced by the Firestorm can affect the
Hephaestean, as can any physical mutation or other effect. Any people witnessing a Hephaestean during a Firestorm can
become afflicted with Disquiet towards the Hephaestean. The Hephaestean does not suffer from Torment- the
Hephaestean already suffers from their own obsessions.
Looks good. That said, nothing has been defined about the Hephaestean's obsessions.
3. Hephaesteans may purchase Transmutations at double cost. Transmutations must have some physical component
(henceforth device) for use: an elixer for enhanced strength, a mini Tesla Coil to shoot lightning, etc. Depending on your
ST, you might be able to art or spiritual symbols as devices, in keeping with the Galateid and Riven progenitors.
IMHO, a Hephaestean's Transmutations represent the knowledge of how to make the relevant devices and concoctions. One of the drawbacks of being a Hephaestean rather than a Promethean is that you actually have to make the device or concoction before you can use it.
The difference between devices and concoctions is that concoctions are easier to make, but they have to be made anew for each use of the Transmutation. Devices only need to be made once; but then they require an Azothic power supply of some sort in order to be used. With a concoction, the necessary power is infused into it during its creation.
3a. They may hold a number of Pyros up to their dots in Composure, and must use these points to activate their
Transmutations. Pyros cannot be regained in a similar manner to Prometheans: without Azoth, mortals must -steal- their
fire through obsessive research or experiments. Hephaesteans may also regain Pyros from every interaction with the
Created (absorb Azoth "radiations"). Roll Composure, and add successes as points of pyros.
-or-
3b. Spend a willpower point in order to activate the object, representing the devotion of the Hephaestean's obsession to
channel the Divine Fire into the object to power the Transmutation.
I'm leaning toward 3a, but without the innate ability to regain Pyros from interaction with Prometheans. At the very least, there should be some procedure that they need to perform. OTOH, I'm also thinking that Hephaesteans may not have an internal reserve of Pyros, but might instead have the ability to construct "storage batteries" of some sort for the stuff. On the downside, said batteries probably would inflict a Wasteland on their surroundings as well as attracting Pandorans' attention.
Hephaesteans might even have a trick or two of their own, such as being able to surgically extract Vitriol, Pyros, or even Azoth from a being of the Divine Fire (usually a Promethean), at increasing difficulties. The latter, in particular, could be used for their more exotic creations (clones, beast uplifts, etc.)
Vitriol is an interesting dilemma. For a Promethean, acquiring Vitriol is done by completing milestones on one's Pilgrimage. IMHO, Hephaesteans ought to be able to develop a similar "internal reserve" of Vitriol; for them, [i]that[/i], not Pyros, is the (long-term) byproduct of obsessive research. Unlike Prometheans, a Hephaestean's means of acquiring Vitriol is much more open-ended, along the lines of how Arcane Experience works for Mage.
It may be that the only way for a Hephaestean to acquire a new Transmutation is through the use of Vitriol. In effect, Vitriol deals with "internal alchemy", whereas Pyros deals with "external alchemy".
A relationship between the Hephaestean's madness and Vitriol should be considered: on the one hand, acts of desperation are needed to acquire Vitriol; on the other hand, it might be that Vitriol expenditure is a leading cause of Derangements… Or not. I don't know yet.
4. Hephaesteans generally don't have Lineages or follow Refinements. However, they do have methods and motivations
for their obsessions. For each Hephaestean, pick a Refinement that matches the method and motive as close as possible.
Dr Frankenstein wanted to raise the dead simply for the sake of science. So, for the purposes of Transmutation costs, Dr
Frankenstein would be considered to follow the Refinement of Mercury. Geppetto, motivated by desire for a son, would be
on the Refinement of Gold. Picking a Lineage-affinity is a matter of deciding which Lineage's generation method most
closely resembles your means of creating devices.
So having an affinity for the Frankenstein Lineage would result in electrical devices; a Nepri-aligned Lineage would have a decidedly more alchemical bent; and so on? Hmm…
I'm less sure about assigning Refinements to Hephaesteans.
As well, there's the matter of Athanors. IMHO, every Hephaestean who does not already have an Athanor ought to be able to acquire one. The exact mechanics of doing so still need to be determined; but it should be no small task. The benefits are relatively straightforward: the Hephaestean gets to spend Vitriol to improve the associated abilities, and he acquires the Athanor's Redeemed Boon.
5. Devices can be used on others, resulting activating Transmutations on other targets. Similar to the alchemical
solutions from Ordo Dracul and Tome of Mysteries.
Oh, definitely. One of the few fringe benefits of the "externalizing" of the Hephaestean's Transmutations is that it's easier to share them.
I'm also thinking that there might be other things that a Hephaestean might be able to do with Transmutations that a Promethean can't - for instance, he might be able to create a Vitriol-infused concoction that grants a Transmutation to the Promethean that ingests it. This should be pricey: at least as much Vitriol as it would take for the Promethean to buy the Transmutation out of sequence and without the benefit of a Refinement. Humans (including Hephaesteans) wouldn't get anything out of such a concoction.