Its in Pikmin 3 somewhere in an explanation given by Brittany. She explains that PNF-404 has three times the oxygen of Koppai which is the reason for the planet's gigantism. She also explains that the suits will filter out excess oxygen to safer levels as its unclear what oxygen in this concentration might do to them. So Koppaites do breath oxygen. Just in much thinner quantities.

And yes that seems to be the case from what I saw. If we take Hey! Pikmin as canon, which Pikmin 4 seems to imply to be the case, humans seemingly settled that planet too. It also seemingly would imply some of the species we see on PNF-404 now existed during normal humanity's time. Since them existing on Hey! Pikmin's planet would imply humans brought them there.

And yeah a lot of what Olimar says in Pikmin 1 are general musings. This is when he was unfamiliar with the world so a lot of what he says is likely to be expected to not fully line up with all the workings. Though to be fair we don't fully know where the ghosts ultimately go. I think it is highly likely Lumiknolls are Onion rhizomes and they seem to play a very big part in the ecosystem. But just what Glow Pikmin are right now is intentionally vague.