I just pulled this unfortuanate happenstance. What do?
Should I inherit the weapon onto my neuter all Hnowi and hope for another hone fliers? I don't have another good red mage apart from Arvis.
Is she worth building as is?
Tough call. Speaking from personal experience, -ATK Tharja is pretty dang hard to work with. Her res is naturally low, so she depends on that attack stat to survive... and when that stat is a wimpy 29? Yikes.
Even with regular buffs from Eirika or Ephraim, I think H Nowi will make better use of her tome.
I don't have Hone Fliers but Fortify Flies does well enough for my S!Corrin. I'd pass the blade tome on to H!Nowi.
As for Tharja... She can work at -ATK with a buffed blade tome but +HP is entirely wasted on her. If it were -ATK +SPD I'd say give it a go but +HP is very meh.
Neutral HNowi is a definite choice. You'll get much more mileage out of her, even if Nowi only gets normal +4 buffs. Try for another hone fliers as well.
Alternatively though, if you're still pulling on the halloween banner, don't inherit the tome to Nowi just yet since you might get one with better IVs, so you can use the neutral one for hone fliers.
I think the last part of this is really the key - do you think you'll keep pulling on the Halloween banner? If yes, then hold Tharja until you're done. If not, go ahead and give H!Nowi the blade tome. Until you get another H!Nowi or Hinoka, you can use regular Hone Atk or Spd and/or rallies to supplement Fortify Flyers.
I would save her for inheriting her tome to another red mage, when you pull a decent one ?
Katarina would be a perfect choice, a Leo could work too.
HNowi can use it alright thanks to her spread but there are other builds for her, particularly if you are struggling with Hone Fliers :)
Pokemon GO
Fire Emblem Heroes
Fate Grand Order
Dragonball Legends
Dragalia Lost
IV Calc
Heroes