So I've begun to really start thinking about this and there are some things that just don't match up here:
Zenn-La was destroyed in 1947 by the Other. Galactus created such an elaborate illusion of it's existence that the Silver Surfer or no one else would ever know what truly happened...
But, if Zenn-La were destroyed in 1947, in the presence of Galactus and the Silver Surfer (and the Surfer's future self), then why did Galactus return and absorb the planet's energies after the Surfer betrayed him? The planet had already been destroyed long before this event. Why would he create hardships for illusions? Why would he give them the chance to escape if they weren't even real and he knew it? Why bother at all?
We can assume every event played out in the comics with Zenn-La and the Zenn-Lavians was after 1947, as it all involved the Surfer being freed from Galactus' service. So...
Did Fennan Radd and Yarro Gort, among other Zenn-Lavians, ever even truly exist? Yarro was never killed or captured. Where is he? Just an illusion running around Earth?
How could Norrin give some of his Power Cosmic to Shalla Bal, if she were only an illusion?
How could Mephisto take the soul of Shalla Bal if she were only an illusion?
Did known Zenn-Lavians such as Lightwave leave Zenn-La before 1947? He would have had to, or else they aren't real either.
How could a being like the Great One take the entire population and the planet into his mind if they were only an illusion?
It seems to me the whole thing was never really thought out. It's a shame. There has been no mention of this plot line since. It's as if Zenn-La and the Surfer's love Shalla Bal just don't exist anymore. I hope one day they address these issues and, as much as I hate when they do, retcon the hell out of this. It makes no sense and seems to be just writing with no research done to make sure there were no contradictions...