Rolling too many Nowi's so...

Sure, make 1 Nowi 5* to get Lightning Breath+, then merge that one into a 4* one and make a 4* +10, if possible. Although you might want to wait until you have more Nowi's to do so.

No. It's not worth 4 star +10ing your Nowi. You'd be losing out on so much by doing that. 5 star +10 her. It helps so much in arena to have a unit like that. Grind free hero merit with the grand hero battles and you can earn those feathers. It's worth putting the time in to get a really valuable unit.

I'm with Vash on this one. Some units are fine to 4+10, such as Cecilia. Nowi however, has huge value. Being a blue dragon, she is practically the only dragon that can take on Falchion users. Her stats and builds in general turn her into a very reliable tank. She's also a great core unit in general for the Arena. Pretty much the only expensive skill (in terms of rarity) for her is Steady Breath, which requires the sacrifice of a Brave Ike.

Feathers can be scarce, but in all honestly it isn't too difficult to get a 5+10 in my opinion. At minimum for a 5+10 on any unit, you're looking at 200,000 feathers. Per month we can earn roughly 75k or so. With the new modes being introduced (assuming Grand Conquest returns soon) and possibly there being more feathers in the upcoming TT+ tonight, the monthly feather count is probably closer towards 100k per month. That's not counting sending home units or grinding hero merit or the inevitable Voting Gauntlet (we just had one so it should still be awhile until we have another).

Personally, I wouldn't start any merge projects unless I had almost enough feathers and almost enough if not all bases required for a max merge. Once you have enough characters to hold a decent enough Arena rank (a few 5* GHB units since they are now part of the weekly bonus and possibly the Askr trio, plus SI), then you should probably put more focus into a merge project so that you have a nice guaranteed income of feathers.