I really hate the idea of making +1-3 merges on units with uniquely inheritable skills, don't even bother unless you're going to whale for a +10. The benefit is so marginal in both power and BST increase.
That said, I would give Steady Breath to a +atk Nowi if I had a spare B!Ike. Now there's a real counter to the power creep (as in all reds+ bows)
Don't forget to add in the DD seal. I've run into it in Arena a few times, and it's pretty annoying. Try the Lunatic challenge map #4 that went up today to get an idea of what it's like. It's probably the unit that has most made me consider moving to a Firesweep Bow build for Bowlyn, but it nukes her Arena score and teams with her are already pretty hard to keep in tier 20 right now without some heavy merging.
The new challenge maps today reminded me how much I want to build a beastly Nowi. The problem has been a) I can't seem to get an ideal Nowi to promote and b) despite 4 focus banners, the game refuses to give me a Ninian.
Anyway, I have 7 4 star Nowis. Here are the IV options:
-Spd seems like a horrible idea on her. I had been holding out for a -HP one with an attack or speed boon before investing.
+atk/-spd for sure, one of her best boons/banes in my opinion. People used to like +spd to put her at 30 which was considered "safe" once upon a time, but that time is gone. Now, there are so few opponents who seriously threaten nowi who won't double her at 30 speed or 1-shot her anyway. And since you're not going to be supplementing her speed via skills, you should just tank it and build her to survive what she can.
The choices for her B-slot are swordbreaker, lancebreaker, bowbreaker, or QR3 + DD3 seal. She'll be vulnerable to bladetomes and quad builds running desperation, but she hard counters a lot of what she's built to counter.
Hmm, yeah I played around with that build on Kagero Chart and it's pretty beastly. Of course, it'd be a lot easier to run if I had ever gotten just one Ninian. ;_;
I need 5k more feathers before I can promote another 4 star (went on a spending spree recently). So maybe I'll consider that for Monday whether I give her SB or not.
Steady breath is good on every unit you listed, it comes down to which one you'll get the most mileage/enjoyment out of using. Steady breath QR on black knight is pretty beastly, the main reason I'd pick him over lancina is cuz she's generally a support unit rather than a baiter. But if you like using her that way then I mean yeah can't really go wrong with steady breath, it's just that good.