Should I give Halloween Myrrh Lightning Breath until I can give her DC?

If you’re planning to use an, only, enemy phased H!Myrrh, then go for it. Seems like a nice budget option, DC would work well with Special Fighter.

I would wait for Distant Counter and use her Armor March for mobility paired with Armored Boots or Distant Defense 3 Seal. Her breath attack guarantees a follow up attack if she has 5 or more defense against the enemy and she can take a hit both magically and physically, so she can bait an enemy for the next turn.

No. It's not worth it. They only way it would be better than her base kit would be if the build was Lightning Breath, Warding Breath, Bold Fighter, Quick Riposte seal. Pretty expensive for a "for now" build.

Her prf gives her the ability to double on both phases ignoring speed. If you remove her prf, you need to apply two different 2 skills to get the same effect.

For my Myrrh, I just used her without DC. It is not difficult. Ranged units are very rare in arena and she can almost always take a hit or two and then kill on player phase.