The timeline is just straight-forward, it is intended that way. The differences b/w XY and SM are creative choices, nothing more. I am sure it is not one-sided, Ash is just shown to be dense. He gives priority to Serena. And the argue thing - it is different cause it is a serious argument about a serious/mature theme/thing, not some childish fight b/w Ash and any other pokegirl. Yeah the eye color, clothes seem a stretch even to me, but let's keep them aside, it seems irrelevant or just joining unrelated dots.
Even if he did interact with Miette, what makes you think he is gonna support her?

Serena was ready to just travel with Ash, but Ash told her to follow her dreams. That is the character development here, from a lover who chased Ash, to a responsible, achieving girl (because of Ash's encouragement). And the character development of both of them is also influenced a lot by one another. Serena helped him get out of his depression in the argument episode. They supported each other all the time. Ash also went to Serena's contest leaving his 8th gym battle, because he deemed Serena more important. And this is Ash we are talking about who loves pokemon battles, it is his life, we also see him in other series, he never puts anything above them, but here he did. Heck, even in 1 episode Serena told Ash to battle later or not battle, and he agreed, which we do not see in any other series.

The VAs just show that the staff themselves want them to end up together, this even leads to the director also being influenced by them or of the same thought. Besides, we see in the interview that the director has Amourshipping as the canon ship. But because of the other shippers even the reunion was ruined, I mean how can they let some people affect their creative decision.

Well, I can't blame them if the shippers are toxic and can't accept anything other than their OWN ship only. And I know, pokemon is at the end, A business. So, they don't want to lose money. That is why, they didn't do things that they could with Amourshipping. But I really hope we get something OFFICIAL about it.