1. White Mage is Conjurer. The two aren't desegregated just because of a name change.
2. If we're going with elements tied to Light then White Mage would have Ice. That's the element most strongly associated with Light/Umbral in the game, hence why Shiva in Eden turned out the way it did. So even going off of that reasoning it doesn't really line up.
3. Flare... isn't Dark Magic. It's Fire. The mastery of Black Magic (for a time) isn't Darkness, it's Fire. Lately Black Mages have picked up some Darkness-like spells (arguably the unaspected spells are those presumably), but the bulk of their theme isn't really Darkness stuff either. And they use the element most closely aligned with Light as well, Ice. So if you try and look at it from the XIV lens of six elements and two polarities it still doesn't quite line up to Black Mages being dark masters.
4. As far as White Mages not fitting the theme of... I'm not sure I quite understand the argument, I guess XIV's elemental balances? As far as that goes, the two jobs are not balanced around the elemental wheel. Black Mages use the three standard elemental spells they've always used, and White Mages got a nature theme added to them and used a couple elements they've used elsewhere with a third added (Earth (though thinking on it Mobius does use Earth I believe)). There isn't anything wrong with it, but it isn't about balancing the wheel of elements at all and astral/umbral polarities. Black Mage bounces between the two poles as is, it's literally in the name. More to that point Black Mage is if anything more light aligned as its Umbral element is the peak light one, while its Astral one is Fire not Lightning. So looking at that kind of angle shows a really weird system in place.
5. In short, the elements don't really flow that way. Since they don't, actually having White Mage move to a proper Light theme (which is the closest to a consistent theme with the job) is better. They can always take those three elements and flesh them out better in another job and make that more of a theme. We could even see a proper Dark themed spellcaster at some point too, though I don't really expect that. To me Black Mage and White Mage aren't really opposites so much as a hold over from FFI being inspired by DnD. At least in terms of elements they aren't or in terms of themes. That said White Mage has been focused on the divine/holy much more consistently than anything else.
6. Devout, Geomancer, and Time Mage all can stand independently of existing jobs. Especially when one need only look at Scholar to see how much a job can change from past incarnations. That said I don't really see any point in Devout when White Mage does what Devout presumably would do. Geomancer stands out as a solidly nature themed job and in terms of mechanics could have a zone based gimmick perhaps. Time Mage is magic that manipulates time and space (including Earth, which could be a thing for taking away it from White Mage), Astrologian manipulates fate and uses star themed magic. While they overlap with Gravity it's not too heavy of an overlap really. Really it's just makes a case that Earth is much better suited for Geomancer or Time Mage than White Mage as White Mage has a solid theme that works without Earth, while Geo/Time would have a much harder time pulling it off.