which of my 4 units should inherit heavy blade seal?

my first answer went straight to Effie, but you arent a brave lance set, and those are the types of sets heavy blade works well with. so if i were you right now i would put it on Ryoma, due to his high attack, and the fact that he is the only unit there that can double. If you want to put heavy blade on a brave lance Effie thats also fine, but you already have DC so its better to just use that.
If you want to build up a brave Axe Cherche i would go for that because right now she is one of the best candidates for it (especially with galeforce and hone fliers) because she has one of the best attack stats only outclassed by Effie and W!Chrom

Either Ryoma or Hector. If you use it on Hector, he can proc Ignis in one round if he is doubled, and on Ryoma you could proc draconic aura in one round if he doubles. Effie needs Distant Defense to continue being a solid mage tank and Dorcas is outclassed by a Hector with that many merges.

All of this Heroes can make good use of it. But i would dodge it on Effie because think you want to play with Wary Fighter with her.
With that you have Dorcas and Hector with you can always ensure in lure QR that they do 2 hits should be the best use for them with Heavy Blade Seal.
Ryoma would it be more a offense option still ofc a good lure Hero too.

Would rather say depends more on how you want use special example Ryoma could make use of it with moonbow now like instant attacker if played with desperation but i think Luna if not Aethir would be better for him cause of most in time as Lure Hero combi with Heavy Blade.
Hector/ Dorcas with their special would be fine and ensure to proc it in first fight most in time still there would be a option to change to Aethir thx to Heavy Blade and would actually incrase your BST in Arena.
On other hand Ryoma and Dorcas have more mobility vs Hector still your Hector has the best Bulk.
Depends all of your playstyle.

I'm going to also put my vote in for Ryoma. Both Hector and Effie prefer their enemy phase seals over Heavy Blade in most scenarios, and as high ATK armored units they don't need to charge their specials for a kill most of the time, and Ryoma's high speed lets him abuse Heavy Blade on player phase more effectively than Dorcas.

Ryoma can work well if you have a spare Wrath and Aether, it'll make him an incredibly hard to beat tank and the quick Aether charge paired with the +10 Wrath damage would be incredibly strong in both player and enemy phase. Alternatively, you can use Luna/Draconic Aura if you don't need the healing from Aether. Otherwise, I like your Fury + Vantage set.

Another minor suggestion, why not try giving Effie Atk Smoke seal and Hector DD seal? (Excluding 500 SP specials) That'll get you the most amount of Arena score, and Atk Smoke Effie can help your Ryoma charge his special.

i say hector because then you could have him firing off ignis' instead of bonfires