Well, the respec NPC is at the camp in Act2.
Level cap is 20 + 10 Mythic Ranks (actually 9 for practical purposes). Difficulty has no impact here.
Dragonheir Scion is like a worse fighter - which is a rather vanilla class anyway. But if that's what you like...
This is incorrect.
DH Scion is an archetype where you don’t see all you’re getting until you make a choice (of Bloodline, like Sorc or Bloodrager) within character creation. It’s the same mistake people make with Hellknight/Cavalier Orders.
What you’re looking for in an archetype are unique features that change the way the game plays out without giving up the unique strengths of the class. DH Scion has two:
(1) It’s the only way in the game to get Dazzling Display as a Standard Action without a penalty *and* without burning a Mythic. Opening Combat with Move plus Dazzling is incredibly strong from lvl 6 (when you get the ability) to lvl 15 (earliest Frightful Aspect is available) or later for party comps with no access to Frightful. Before then you can just use Demoralize to do same on Single Target.
The ability itself saves you a Feat, not needing Weapon Focus saves another and works well with Fighter Martial Prof to use best available, and the class comes with a scaling Intimidate bonus. Fighter Bonus Feats make Intimidating Prowess and Skill Focus Persuasion more affordable as well.
(2) It’s the only way in the game to get Arcane Strike without Arcane casting ability and you get it at level one where it does the most relative damage.
IIRC it doesn’t progress (because you don’t get Caster Levels) but the Bloodline does unlock Elemental Damage on it and it’s a prereq for PnP Riving Strike which is available from several common mods. Arcane Strike applies to every hit, not sure if this stacks.
The Bloodline itself offers scaling Natural Armor Bonus (stacks with Barkskin), Elemental Resistances, and Wings at lvl 15. The AC benefit here is likely larger than base Fighter would get from Armor training.
Losing Bravery doesn’t matter much if you’re getting Fear Immunity from a Paladin.
With Scaling Weapon Training, Intimidate, and Natural Armor Bonus the class works well on its own (Capstone gives Immunity to Paralysis, Sleep (Phantasmal Putrefaction), Damage of the Bloodline Energy Type, and 60ft Blindsense) but (1) and (2) above are already there at lvl 6 if you want to use it as base for Prestige.
Sets up well for Dragon Disciple (needs one lvl dip in Arcane casting class) to fight in Dragonform without having to cast it since its full BAB and Weapon Training is nice boost to Dragon Natural Attacks (as is the extra Natural Armor). Not sure how Bloodlines interact.
Pure DH Scion can also fight transformed from Demon or Beast Shape spells from another caster in your party.