Best refinement for Nowi and Res Ploy

I'd go with either Defense or Resistance (most likely resistance). I feel like those two are the most important when it comes to dragons since they're mostly used for Enemy Phase.

As far as Lute is concerned, I'd give Res Ploy to Micaiah.

I guess it depends on what kind of role each would play. I use both Winter Tharja and Micaiah on my team, but I would rather Tharja have a armor buffer cuz the rest of my team is armor.
They are both very good candidates for this skill! Micaiah could score more KOs, but Tharja has the cardinal direction advantage against melee enemies. Being an armor, she can take hits and retaliate tenfold.
So maybe just wait a little after doing some Arena this week? That's what I would do. Unless you have a clear idea of how you want to use each unit.

As for Nowi, Threaten Res is fine since she wants to affect foes in all directions that she can counter with Lighting Breath.

Also, I think Nowi's speed should not be refined, maybe go for Res? The threats who would double her are probably fast green mages who would double her anyway. Making her a balanced tank lets her take hits and retaliate with extra power on ranged foes with superbanes. Not to mention all the low res red swords out there.