So you're saying that even with a timeframe long enough that humans who weren't born yet when they started would have been dying of old age before the deadline, and access to the wealth of Heaven as well as all the exotic plunder from the Primordial War, with all that, the group of 30 solars (for whom that charm, by your own argument, IS a logical purchase), their respective lunar mates, two perfect circles of Sidereals, a proportionately enormous group of Dragon-Blooded, allied People of Adamant, and possibly even Autocthon Himself, couldn't reasonably have built a particular 2-dot artifact? Actually, if it's either crystalline or plant-based, one of the Dragon Kings with the right Path could reduce that to effectively a 1-dot. Making it plant-based biotech would also apparently resolve your concern about compatibility with the Imbue Amalgam spell.
Per the training time rules in the core book, inventing a new charm from scratch takes weeks, not decades. The Ochre Fountain Era wasn't exactly peaceful, but it was a time of prosperity and expansion during which the Exalted Host had more than enough time to explore the more tactically useful 'esoteric' possibilities of Essence 4 and build basic artifacts.
Infernals (well, actual pre-yozi primordials at least) were relatively common opponents during the Primordial War, which had only recently ended, and Unshaped might have been far more common in the immediate aftermath of the Three Spheres Cataclysm, or when pushing out into the Deep Wyld or Pure Chaos when those bumped right up against the borders of the world. Lunar tattoos definitely hadn't been invented yet. If somebody slipped up and didn't keep their Shaping defenses active, wouldn't it be nice to have a backup plan, a second line of defense? An ounce of prevention is great, but any smart pharmacist will keep a few pounds of cure on the shelf too, because accidents happen.
I'm not talking about mixing the two, or casually swapping out one for the other, but rather designing an entirely new spell which uses Autocthonian influences to achieve a mechanically similar result.