Yeah, I read that literally at first, i.e. Flowery was somehow, literally, Asgore's old roommate. By the end, my reading was that it was a convenient fiction for the fantasy world they concocted. With Asgore aware of that and the nature of the Dark World, he was just "playing along". The Wiki says Rudy and Asgore were roommates so maybe Flowery's personality is based on his?

Leaning into authorial intent, I think everything about Flowery will exist only in Chapter 5, but it's open-ended enough to be anything convenient for lore later on.

But I think the flowers want to be humans in paralleling how their SOULs came from humans in Undertale. It might be related to the dark and fateful day Asgore vagues about.

With you on the mysteries. I was big big big into Undertale lore, we still don't quite understand how the Delta Rune from Undertale relates to Deltarune, or how DETERMINATION works here. But the fact that Flowery did not de-terminate makes me think, hey, isn't that explicitly what the golden flower is supposed to do?