Which Cordelia?

Well, I also was cursed with a Cordelia of +atk/-spd and I don't plan to 5-star her since having any other bane is better for her, but the +atk still makes her brave lance gain a solid 6 damage (3 per strike) for most enemies and with a penalty to her speed, it's less tempting to rune the standard L&D quad strike build so you can actually go for something safer most likely.

haven't had time to mess with her myself yet but here's what I'd use for her if you are keeping her 4-star:

Cordelia
+atk/-spd
Brave Lance
Reposition/Swap
Escutcheon/Sacred Cowl
Attack +3/Fury
Flexible/Hit and Run
Flier Buff (Prolly Goad)

The a skill depends on if you have a bunch of Hinata laying about and if you do, you can amp up her bulk to decent levels and still give her solid attack as wll, but it limits her longer lasting power without a healer to support her.

Reposition vs Swap is a choice you make by preference since they're both Flier Emblem staple assists.

The defense special is chosen because 1 initiation with Brave Lance has it charged every time to protect her on the counter (if they somehow survive) or the first attack on the enemy phase, helping keep her in the battle longer.

Goad Fliers is to my knowledge the most commonly spread buff because of all the position play but if you want to, Ward is likely fine too.

B slot for fliers is usually Hit and Run or Draw Back for budget choices but if you want, Vantage or Quick Riposte 2 could be amusing to mess with too.

I have a +atk who I am going to invest in. With your +atk, if you give her Death Blow and Hit and Run, -spd shouldn't be as much of a problem (even better if you have Hinoka/Hone Fliers). If you do want to quad with her, go with neutral and LnD

Go with +atk, absolutely. +atk is critical on Brave weapons, and -spd does not cause nearly as many problems as you would think. The optimal variant of Cordelia, or any brave unit, aims to annihilate the enemy in one hit. With death blow and a +atk nature, this becomes simple, not to mention further boosts. Cordelia should not be taking hits, as she is purely a damage dealer, and therefore speed is nearly irrelevant. Look at Reinhardt, or Cherche's brave axe build - those units have abysmal speed, but you don't expect Reinhardt to take a hit, or to double anything - you just expect him to annihilate something on initiation. You should be keeping Cordelia out of combat through a combination of draw back/hit and run and dancing/positioning skills anyways. I hope this helps. Obviously if you're taking off her Brave Lance+ then the situation changes, but I see no reason to do that.