The thing about the relation of Zygarde to Solgaleo and Lunaala is that it is purely FAN SPECULATION. That is NOT confirmed information. Until GF and TPCi decide to reveal that information, it's hard to say for certain. They could very well be for Sun and Moon legends, but they could also be an aesthetic thing since the shades are just lightening (Dark blue to light blue to white, and red to orange to white). Also, Zygarde Complete also features predominant RED and BLUE colors in its body, associating it more closely to Xerneas and Yveltal, which while obvious since they are the KALOS Trio, it would have made sense to include the orange and light blue colors as well if Zygarde was supposed to be related to Solgaleo and Lunaala to a heavy degree (Although, in the moment thinking has me wondering about the shiny coloring... maybe the red and blue appendages will be orange and light blue in shiny form?).
Yeah, skipping Azure Flute and Z are two completely different scenarios, but also both relatable. They could very well have scrapped a Z project, and also very well given us the XY&Z anime, the Zygarde promos, and all the green focus JUST to throw off fans in anticipation for Sun and Moon.
The issue I personally take with this Zygarde business is that so many people, here and other forums/tumblr/youtube/etc. seem to believe whole-heartedly that a return to Kalos post-game is a necessity. It's not. It's just not. The reasoning behind these claims I've seen? If it wasn't part of the "Kalos felt unfinished because 'Southern Kalos'" party, then it's the "But ZYGARDE" party. Going back to a region we JUST finished not too long ago just for ONE legendary Pokemon does not make the most logic. In fact, I find it to make the least logic. The idea of another Zygarde being in Alola makes more sense than a return trip to Kalos to finish Zygarde and its forms.
I'm honestly just waiting for these games to come out so we can finally put it to rest. If they DO include the forms and stuff, then that's GREAT! FANTASTIC! People should just not be shocked and feeling cheated because Zygarde doesn't get its own post-game Kalos return trip of anything remotely plot/story related. They shouldn't hold hope out that Zygarde is somewhere on the islands waiting to be captured, and if things like the cells and cores aren't available. Same all goes for Ash-Greninja too.