Hina vs. Viera isn't actually a direct comparison. Viera, as a Sailor, is a gatekeeper, you activate her skill to deliberately let specific enemies pass, not to do ranged damage (though if you have, say, an AW Heavy in front of her, it's not a bad idea since it's unlikely you'd need her block at that point) while Hina is a ninja, constant range damage. If your focus for this choice is ranged melee, Hina wins easily since Viera only has range during skill.
2nd question was answered? Well, hoping answer was Thetis because she obliterates Emilia in every category except during skill (and cost, obviously) and her AW ability, while niche, helps a ton when that niche is relevant
3rd: Elva's cost is rather prohibitive without using Reinforcement Soldiers, and maybe Valkyrie UP ramp on top of that, but she's a monster if you can deploy her consistently when you need the 2nd tank.
If you're looking for where to focus, I'd HIGHLY recommend pushing Aisha to 80 and AW her for that lovely Team HP Up, since she's already 78. Adele would be next, for a smaller HP Up boost that stacks with Aisha's, followed by either Katie or Zenobia (I'd go with Z since she's stronger fighter, can use Call for Reinforcements earlier, and Katie's 2% Defense Up team boost isn't much of a difference except on tanks like Bernice and Elva). Probably raise Minerva, Sophie and Elva with your black spirit/platinum armor combos if those are available. Lilia and Imelia are good choices for dueling, Lilia's already waiting to be AW, and Imelia's at CC50 so not too bad either.