Best use of distant counter

Agreed. Having the ability to kill Reinhardt in the same turn should not be understated. By the way, Steady Breath does not cost as much SP as DC does. So even from an arena weighting standpoint, DC works better.

You can get by without Armor March but it's much more challenging. It essentially requires a hybrid team or running Distant Counter on multiple armors - which was more effective before B!Lyn. If you have Tana, a hybrid team of three armors and a flyer with Guidance is perhaps even stronger than Armor March.

Without armor march, you definitely need DC on your armor units, otherwise, mage units can chip them to death while repositioning themselves to safety after the attack.

This is precisely why armor march was revolutionary for armor emblem - it allows armor units to run alternative skills for their a-slots, which were previously reserved for DC. For example, Brave Lance Effie can have Deathblow in her a-slot, which adds 12 damage when she initiates combat.

In other words, it is up to you if you want armor march. If you don't, then DC is kinda a must-have. If you do, you have more flexibility and probably have deathless runs much easier.

Considering most Effies run Wary Fighter, Effie with DC will not only counter red mages but also blue mages. The problem, of course, is that Reinhardt usually runs Lancebreaker to counter precisely that.

In armor emblem, I believe that it is best to have one Distant Counter user of each color. For the reasons that you outlined Green is the most valuable color for Distant Counter but I would prioritize Red/Blue over a second Green. Red mages are uncommon in the current meta but not unheard of – I’ve had a few deathless arena runs ruined by the rare Celica. Since the Black Knight will have DC on his weapon, there is no reason to put DC on a Red so Effie takes priority in my opinion.

Beyond that… Due to her impressive HP and ATK, Effie running Wary Fighter can survive the vast majority of mages (even Greens) to either OHKO them with Distant Counter or to weaken with Distant Counter before finishing them off the next turn. The only mages she really fears are Triangle Adept Greens (most of whom still cannot OHKO her), highly buffed Blade tomes (the AI doesn’t take advantage of this), and an optimized Reinhardt (this is the troublesome one). Distant Counter also allows Effie to charge and deploy her Special more quickly - which enables her to destroy Greens that would otherwise check her.

I think he'd look happier if he had Threaten DEF or Panic Ploy in his C slot.

He's a hundred percent in there, no edit. I wouldn't forget my main boy.

And by the way, I'm loving both the build and the dedication. Keep up the good work mate, looking forward to the complete build!

Edit: you know what? I'll even bump him up in the list :)