Few questions

1. Your P.azura is fine, you don't need to 'improve her ivs', just give her standard moonbow TA and WoM or lancebreaker if you want her to be your hard check to blues. While +spd is nice, dancers don't need those optimal ivs to be good.

2. The seals to put on her depends on what you want for your team. If you want her to support more, then those new spur atk/spd seals seem pretty good. If you want her to be tankier, then go hp/def/res, etc. There's no best option, it depends one what seals you have available and what you're already using for your team.

3. Olivia has more offensive potential if you put poison dagger on her, though her BST is really low so it will hurt your arena score. Ninian can be a good check to reds with TA, and she's tankier but melee (though you can put lightning breath on her for distant counter). I would simply suggest which one you personally like more, dancers are dancers. If there are some blue units you want, then it might be more worth it to pull from blue since there are good units/fodder you might get while pulling for ninian. Takumi as a unit is still fine, just not top tier anymore. The -atk bane will hurt him. Unless you really like him or need close counter for something, I don't think he's worth pulling for.

Honestly those IVs are fine.

To be fun and utilize these new sacred seals, I would run TA, WoM, spur speed and spur speed seal. After dancing an S support unit and being adjacent to them, the ally will receive 13 speed (3 from dance, 2 from support, and 8 from spurs). This positioning combo may seem difficult to pull off, but it is actually guaranteed if the ally receiving these buffs has swap. If the ally is a mage, why not put blade and desperation on him/her. Now, you essentially made a speed creep god.

Just a suggestion.