What do with Veronica?

She has 33/36 neutral. I recommend +atk cause +spd won't really help her since Staff units don't have that good of a offensive weapon to use(until IS introduced Brave staff +atk will be the best IV) and With +atk and if you are gonna keep using her default healing assist skill she will heal more damage.
And depends on what you prefer if you want her to avoid mages or physical units. but IMO go with -def.

If there's hone cavalry and chill spd I think +spd is definitely a strong option. She has, what, 45 spd? And then she uses spd+3 as A-skill. And any unit that pushes into the upper 40s is probably getting hit with a -7, so she has effectively 55 against that unit, which will probably double even most Karlas.

On a mixed team I think I still prefer +spd. If she's receiving a bunch of tactics buffs, then it's the same as hone cavalry; if hones or drives are going around she can still achieve spd well into the 40s and maybe even effectively 50s against a unit hit by chill spd. She's fried if she ever gets hit twice, and she's probably never going to bust anyone without a double. +atk with atk+3 as A-skill gives 52 atk, which most units can survive once. 46 atk with 42 spd probably nets more one-rounds.

Between her two super-thin defenses I think I prefer to drop her def because that way she has maximum ability to survive one hit from anything. If my memory serves, she also has a superbane in res, which would take her down to 51 which is definitely OHKO territory even for mages that don't hit super hard.

As for comparison with Maribelle, this is an easy one. She has Maribelle completely outdone due to her ability to inflict massive debuffs on the enemy team while potentially buffing her allies by the same massive amount. She also has windfire balm for those times when she doesn't attack (though you might find safer just to put miracle on her).