Sonya

None of those IVs are great but I would probably go with your +Atk / -Spd one and give her QR2 as a B skill. Her natural A skill is fine since you don't have IVs that merit heavy investment.

I would save your spares to fodder for Res Ploy. I also think Mirror Strike would be pretty good on Lute if you happen to pull one. Not as good as Swift Sparrow but not bad at all.

The question of whether to merge always comes down to whether the rest of your unit are merged or not. Since most of my 5*s are +1 through +3, I always merge, even when they have useful SI like Brave Lyn. If most of your 5*s are unmerged, sit on the extra copies for now.

I think either +atk/-spd or +spd-atk. Both have pros and cons. +spd allows her to run the fast LnD3, Spd+3 seal set with Moonbow to slay enemy mages. It also allows her to run a standard Fury 3 res tank set, in which her primary goal is tanking Rein, doubling him, and then loosing a blistering Glacies against the next target for a clean 2-0.

If going with the +atk/-spd set, go Fury 3, Iceberg, GTBreaker or QR3.