Question 1: When attacking with Nier's Blade and using the Iron Hand Combat School, can you use the Deadly Strike Technique?

Question 2: Does the Great Weapon Fighter Fighting Style effect Nier's Blade (i.e can you reroll damage rolls of 1 and 2)?

Interesting questions. I am no one official and I get rules questions wrong almost as often as i get them right so take my opinions with a grain of salt. That said...

I guess the answer really depends on (a) whether or not the conjured sword of flame counts as a melee weapon and (b) if there are special rules for the Nier's Blade. It also worth thinking about (c) whether the combos are bent or or broken compared to other options for second level spells.

(a)The spell explicitly says it creates a great sword (albeit one made of fire) and there is no reason to assume that you couldn't use the sword to do all the things one normally does with a sword ... like take an opportunity attack. So i assume it's a "real" sword and barring something else in the rules it should count as such for other feats and abilities.
(b) In the ARP rule set the Gladius of Light spell creates a similar effect and it has language that specifically bans its use from the ARP version of fighting styles and other mechanics.I see nothing similar here.
(c)I think this spell is potentially better than scorching ray which is the nearest similar spell. But I think scorching ray is probably a weaker second level spell. SR allows you to make 3 attacks that inflict 2d6 at a range of 120' for a maximum of 6d6 damage all at once. Nier's Blade replaces the 2d6 a great sword normally does with 3d8 fire damage on a melee hit. Or essentially an extra d8+2. It's also a concentration spell & takes up both hands which puts limits on further spell casting. At three rounds of use NB definitely deals out more damage than Scorching ray. But you've been sitting in melee all that time probably making a couple of concentration checks. Has the Blade survived? And what other spells did the other spell caster cast in the meantime?
Bottom line, as a matter of making a rough back of the envelope "guesstimate" I don't see anything in Nier's Blade that makes me think it's already too dominant.

Conclusion: As a GM my first thought is that using Nier's Blade with Deadly Strike and Great Weapon Fighter is probably okay. Haven't seen it in action yet. If it seems ridiculous compared to what the Rangers, Myrmidons or Warped Ones are doing in melee, then I am happy to reconsider.

Thank you for the contribution val'Holryn. I had imagined that there was some precedent for Nier's Blade, seeing that it counter as a Greatsword for the purposes of a Cleric of Nier's abilities.