This post is almost pure opinion and theory, feel free to ignore.

Thanks for the info on Scourge+Banish, I guess I never did go deep enough early on in the game. I'd be inclined to believe that the astral/umbral divide of 3+3 elements would be for spells (and possibly other related things) only - in a way, an earthly mage being able to only partially manifest the polarities of a single element, maybe. After all, there's no indication so far that the elements and polarities for gods/months/etc would be wrong, which means there has to be an explanation somewhere for it.

Also somehow neglected to match up elements of the Twelve against the epochs, a 'bit' of an oversight on my part. :X Seems to me there could be a cultural/historic reason between the epochs+eras having their own starting point and the rest having their own, even though it's not mentioned. It's not like RL is uniform either, and that's due to historic reasons as well (when not restricted by nature itself). You have to remember that while Eorzean culture seems rather unified in the present day, we have no idea when these classifications came to be and who (possibly which group of immigrants) came up with them.

On the matter of Fury(Astral) and Lover(Umbral), while at first glance it can seem backwards, there are other considerations weighing in on this matter I'd say. Halone being the guardian of Ishgard, for example, and everything holy seeming to be so important to them, waiting on judgment day and judging others to be unholy and all. Menphina then, associated with the Moon, makes sense being an Umbral polarity. Consider also the portrayal of Fury, bright rage, purifying rage even? Compared to cloying love and its depths of feeling, not always so very pure in origin and method. If you prefer to go there, there's also the Astral polarity elements (fire-wind-lightning) more easily associated with fury, Umbral polarity elements (ice-earth-water) more with love.

Now watch them decide there's a mistake there and all my carefully crafted reasons blown to naught. But this stuff has been there since the beginning, I'm a bit disinclined to believe they would've made such a fundamental mistake in this particular thing. There has to be reasons for the deviations, even though not mentioned.

Also factual point, RE: 7th Umbral Era bringing us back to Wind. Consider the words of Urianger Augurelt in the 1.21 and forward event. This should ease your doubts about the next Era.

Each era is ruled by an element, and of these the realm hath borne witness to six.

Light and darkness yet remain, one of which the Divine Chronicles foretell shall next hold sway.

Ne’er till land consumes sun can sea bear moons,Heavens spew crimson flame, hells seep black dooms.

Will destruction rain down from the firmament? Or will it seep forth from the bowels of the nether?