People freak out too much about IVs. For me, the only absolute killer is -spd on a spd oriented unit since it compromises doubling and double avoidance.
Not really, Units that has decent atk and great spd will still do well with -atk +spd.
with her default kit she will have little worry of not dealing damage to melee weapon units.
Not even in the slightest. Thogn and her kit patches up her attack and takes her speed even higher, especially against melee units. You don't really need an Attack Seal in my opinion, just a Flashing Blade to charge specials like Moonbow faster.
Normally -Atk is a killer, but for L!Lucina it isn't really. She'll still have around the same attack as L!Lyn, the other non-colorless archer. And, your L!Lucina will have more speed to boot
Base kit you're looking at a 53/49 offensive stat line if you initiate on a melee opponent, and that's before any team buffs. I think I'd rather have that than any sort of hit to HP or Def because of her mediocre physical bulk.
Pokemon GO
Fire Emblem Heroes
Fate Grand Order
Dragonball Legends
Dragalia Lost
IV Calc
Heroes