I like your version better, Thoth Lad :-) Any way to also integrate Nemesis Kid's backstory into this? Kind of weird that we have two alchemical bad guys with no relationship to each other.
Alchemy had crossovers with metallurgy and chemistry as well as more esoteric traditions. A combination of science and more mystical pursuits. We're only told that Nemesis Kid had alchemical skills that he used to give him his power. The nature of them is left unknown. That we see Cosmic King use a ray doesn't mean there were not other disciplines at work. Alchemy's influence on mankind has waxed and waned over the centuries. In the darker centuries between our time and the Legion's its influence would rise again.
The thing with Cosmic King is that you can never be sure of where on his timeline you are seeing him. Chronologically, he was first seen in an early issue of the Baxter run, as part of the Legion of Super Villains. But he made his debut in the adult Legion of Super Villains back in August 1961. The origin is the same, with him stumbling into a transmutation ray he was working on, and being expelled by the Venusians. But in that Superman 147 appearance, he claims to have joined the Legion of Supervillains in the 21st Century. Cosmic King has fought as a villain in the 20th,21st, 30th and 31st centuries. He's also been seen as involved in creating parallel timelines (Superman/Batman). I see Cosmic King as a time traveller, searching the centuries for the secrets of alchemy, including immortality. He is the seeker in the LSV, more interesting in the secrets that conquest and time travel bring.
In his quest Cosmic King has become a feature of history, appearing and disappearing in records across the millennia. Sometimes, he would not leave a trace, and in others he would leave a following behind. Believers, with alchemical training and skills, who would, like other peoples, look to find their own niche in the wider galaxy. It was no accident that the effects of exposure to Trom's environment resulted in transmutation abilities for a select few. Those few were the descendants of one of Cosmic King's groups, and had searched the galaxy for such a location.
Another group would colonise what would become known as the Alchemists' Planet, Myar, the home world of Nemesis Kid.
Nemesis Kid's try out involved more transformative abilities than it would later seem. Against the Atomic Robot he transformed the air into lead, and then himself into neutronium (looking oddly like later candidate Ferro Lad) to defeat his foe. He does say that he has the power to beat any foe, but they originally operate through alchemical alterations. At the end of the story he does try to use his power against multiple foes. The Legion assume he has teleported away. But a transformation into a gas would have had just the same effect, and been consistent with his earlier powers, allowing Nemesis Kid to drift away to safety.