Evidence: Nagisa and Honoka appearing at the Mahora Festival in the fourth panel. How did they end there? Well, after the Dark King attacked Earth directly the mages finally tracked the Pretty Cures, and to keep The Masquerade offered them a scholarship and training in Mahora. It would also explain how they became able to one-shot a Zakenna in the All Stars movies: the training paid off.
Futari wa Pretty Cure is indirectly based on events in the past of Futari Wa Pretty Cure Splash Star.We know from Saki's English quiz that some TV show called Pretty Cure exists in the Splash Star universe. According to an early episode, there's been at least one Pretty Cure in the Splash Star universe prior to Saki and Mai. It's not unreasonable that the two might be connected. In the distant past, Dark Fall made an attempt to capture the seven fountains. A pair of women named Nagisa and Honoka, aided by the spirits Mepple and Mipple, fought them off and won, but didn't quite manage to wipe out the bad guys. (Possibly, they never realized Goyan was the real Big Bad.) Their story was known in part by the Muggles and, through repetition, it lost details (such as Uzainas being produced from different things depending on who creates them), and some things got mixed up. Fast forward to the modern day, where some Toei exec decides that a modernized version of the old Pretty Cure "myth" would make a great anime. Nagisa and Honoka were recast as schoolgirls, with the added "complete opposites" angle, and new names were made up for everything whose real name was forgotten. The Hikari angle was probably invented as an excuse to extend the show for another year as Max Heart. (Alternatively, perhaps instead of the Queen being restored to her original form, the real Hikari regained the Queen's power and memories in her current body, becoming the Filia we know.) Since both the legend and the TV show are well known, Flappy assumes Saki and Mai will instantly understand how this Pretty Cure thing works. This is why he's so uninformative about it at first and irritated when they need things explained to them.
Cure Black is a Kamen Rider disguised as a magical girl.Cure Black's voice actress Yoko Honna had a recurring role in Kamen Rider Black.
Cure Black is a magical Silk Spectre II.Same reason as above, only this time Honna was also the seiyuu for Silk Spectre II in the Japanese dub of Watchmen.
Nagisa and Honoka became Heterosexual Life-Partners after the end of Max HeartTheir powers depends from their feelings for each other. Now, in the series Zakenna always gave them at least a decent fight, but in the first All Stars movie they one-shot two Zakennas in a row. My guess is that their powers have been brought Up to Eleven with their feelings...