A few things I'd like to clear:
1. Light & dark have a really meager advantage against the rest of the elements(unless they're up against their counterpart in which case the player side will have elemental advantage): 3%, which is just a part of the calculation, so it ends up even less damage bonus & they will take the same amount of damage from enemy as other elements with neutrality. Their main charm point is that they don't have elemental weakness, so you can use them against any element without having to consider that.

2. Tower is a very faraway feature first introduced at the end of November last year at the DMM side. This means we still have about 9.5 months until then. And you should take into consideration that we're about to have an upgrade to the access to jewels & premium tickets with the next raid event, Barong. Barong & all raid event afterwards will contain an upgraded reward table with overall 3k jewels & 22 premium tickets. Free stuff showering. Then in June, with Illyunkas, advent events will also include 3k farmable jewels. Also ranking will be introduced just before that at union events. Even if you're not in a competitive union, you will likely see some jewels from it if you're in a union which can complete the rewards (and that's recommended, anyway).

3. Tower missions will give you all in all 10k medals if you complete all of them. That's exactly the cost of the highest reward in the shop: SSR weapon limit breaking material. There are bonus medals you get from clearing the floors within a few set intervals of turns, starting from 0~4t with 50 bonus medals to 20~24t with 10 bonus medals. At the end of the event, you will get bonus medals based on your gathered medals.

4. Hime release weapons if dual weapons, can either have a large+small skill, or two medium skills. I don't think there's any tendency to either case.