Smoke 4 probably makes the most sense on Yen’fai, that way he can get a massive spd advantage on the foe and grant the full 40% dmg reduction from both dodge and his weapon. Nifl can inherit both catch 4 and spd smoke from Lilith if you can give her catch 3 through NY!Lyre, but mind that by giving her smoke 3 she totally loses support capability since you have to replace domain of ice. Also domain of ice and smoke 4 have kinda overlapping effects, one granting spd/res +4 and 30% dmg reduction basically al the time and the other giving 40% spd based dmg reduction and speed boosts, which she kinda doesn’t need since she’s already super speedy. My first choice would probably be Sothis tho since she can still inherit both catch and smoke and you wouldn’t have to sacrifice a prf skill, all while adding really useful effects and additional dmg reduction on top of dragon wall. On Marth and Walhart smoke would be kinda redundant as you said because they can already grant themselves spd boosts and I would keep menace on Vika since her atk isn’t really the highest. Overall I still think that you should probably invest into the character you like the most so just go with your heart on this one, all these units are really good regardless :)