+Atk -Spd FH Celica and Alm

Well, Alm's speed is pretty garbage anyway. Windsweep/Phantom Speed is a combo you could still use, just not as effectively. I'd say with his weapon refined for the effect and QR3, you're getting 2 hits from him on offense or defense. Give him heavy blade seal and Aether and he should consistently be getting himself back up within range for his double strikes. You could give him DC if you have the fodder.
Alm will be fine.

As for Celica, I have - spd, too (although +hp, so yours is better). I have her with Darting Blow 3 and a speed seal, so she's hitting 45 speed on the attack with full health. With the chill speed skill, she still does well. I'd give her fury, but I'm waiting to see if I can get her again woth better IVs and fodder this one to Mia.

You'll be able to make it work with both of them.

For Celica, I'm sorry. I got the original Celica at -Spd, but luckily got a +Def -Res one that I still actively use today. Use her with Darting Blow if going offensive or Close Def if trying to beat slow Green units, in which case she rarely needs any Spd at all.

For Alm, I'd give him Life and Death 3, keeping Windsweep and Phantom Speed 3. You're still at 42 Speed there, without buffs, and with Ayra and Mia fading out of the spotlight, that'll do nicely.