Because of the nature of the helix formula, the INT you should stack for them will basically cap your macc on exp mobs without dark arts.
At 75, you will definitely not have capped accuracy against Imps or Colibri in this situation. Before 75, there simply isn't enough INT available to cap your accuracy via INT alone.
2ndly, wasn't proof posted sometime ago that there is indeed an acc increase when under weather and day effect of the correlating element?
If you are referring to Helices activating weather effects 100% without the use of Obis, then you're half right. I've never seen concrete proof that day/weather effects affect accuracy. If you have more substantial proof, please feel free to share.
3rdly: storm spells weren't worthless pre-SCH merits, and they're still not even before stormsurge now. It's just a matter of deciding when they're most useful. If you're a healer with the light obi, failing to aurorastorm yourself is nigh retarded.
+10% Cure Potency from Aurorastorm comes to about +0.4HP per MP spent on Cure spells. If you have a Korin Obi to proc this 100%, then you only have to spend about 100 MP in 3 minutes on Cure spells to make Aurorastorm worthwhile; if you don't, you'd have to spend 300MP in 3 minutes to get your MP's worth out of Aurorastorm; not worth it. Besides, at 75 I rarely even cast 100MP worth of Cure spells; Stoneskin and Regen2 are much more MP efficient.
RDM can attain an almost static 50% reduction to recast time, SCH can over shoot this for a total of 60% reduction under Alacrity (provided you are wearing loafers and keeping dark weather on yourself)
MACC is a joke, any job with access to mage gear can hit close the MACC cap on legion mobs, which is why my BRD/RDM can land slow/para/blind despite having only 150 skill (from /RDM.)