As for the legion stories claiming that krypton was colonized by daxam. How was that claim presented in the stories? Was that shown to be a fact? Or did Lar-Gand/Mon-El mention that as something he believed? Also how do you account for the planet Zoron? Hyperman's home planet.
Mon-El: "I'll let you in on one of the galaxy's hidden secrets, Jo. Kryptonians and Daxamites share the same genetic heritage."
Gigi Cusmano: "You and him [Dev-Em] related?
White Witch: "We are all products of planetary seeding, officer. Naltorians can trace their line back ... to the Sorceror's World, and even to Earth!"
Ultra Boy: "Wow! So, which one was the colony planet, Mon-El? Krypton or Daxam?"
Mon-El: "Well, I suppose we could be accused of chauvinism, but I was taught in school that Daxam is the parent world."
Dev-Em (awakening, recovering from green K-poisoning): "Uhhhh ... No way, Mon-El. I'm not claiming you as my long-lost daddy."
-- Tales of the Legion of Super-Heroes #325, July 1985, page 6, panels 2 - 6
It must be noted that Lar's med-settings were what helped restore a nearly-dead Dev-Em. Said settings made Jo think Mon had been studying Kryptonian physiology when Mon's explanation was he used settings appropriate for their shared heritage (see quote above). The fact that Mon's settings saved Dev's life is pretty safe evidence they are indeed related and of the same species, regardless of Dev's woozy protestations.
Also, Lar was both an astronaut and a Daxamite scientist while Dev was a juvenile delinquent on their respective worlds. I'd be inclined to think Mon had a better education and better credibility on this than Dev.
Zoron was either retconned out of existence by its lack of being mentioned ever again in the stories. Or if it did still exist in continuity, then Zoron might have been colonized by Daxam too. We colonize our entire solar system and many other ones in many future histories, so it's not a stretch at all that if Daxam colonized Krypton that they also would do so to any another habitable world in the same solar system.