DedWards

Evolving Wilds to help with mana fixing.
Runechanter's Pike or Spellheart Chimera to take advantage of the ever filling graveyard.
Mana Leak; and Rune Snag are some of my favourite counter spells.

Good thing I already have a set of Hunted Horror :D

Hardened Scales

I just double checked, just to be sure, and you're right. Now I have even more reason to get Thing in the Ice.

Hunted Horror would be worth considering too. Thing in the Ice won't bounce it, but will bounce the centaurs it creates.

I'm not 100% sure Vampire Hexmage will flip Thing in the Ice.

"Whenever you cast an instant or sorcery spell, remove an ice counter from Thing in the Ice. Then, if it has no ice counters on it, transform it."

The "Then if" part of its transformation has me concerned about the interaction with Hexmage's removal effect. The clause has me thinking that the last counter removed must be done via its own ability for it to transform. I could be wrong though and Hexmage does allow it to flip, just saying that a ruling may be required.

Hardened Scales - boosts the number of +1/+1 counters.
Evolving Wilds - helps with mana fixing
Blossoming Sands - helps with mana fixing
Enduring Scalelord - with two of these out, they can get indefinitely big.
Inspiring Call - card draw and protects your creatures from most removal.
Managorger Hydra - gains +1/+1 counters whenever any player casts a spell.
Sunscorch Regent - gains +1/+1 counters and gives you 1 life whenever an opponent casts a spell.

I'm surprised no one mentioned this. You can double up on Voyaging Satyr with Kiora's Follower. Just like the satyr, it can untap Nykthos, but unlike the satyr, it can also untap Karametra's Acolyte. The Follower even adds the same amount of green devotion.

Don't forget Surgical Extraction to get rid of those things that shuffle back automatically.

The Altar would supply a nice alternative win.

Zulaport Cutthroat and Blood Artist die to Heartless Summoning, that's why I suggested Falkenrath Noble.
I also suggested Krark-Clan Ironworks as you can loop Myr Retriever in the same way with it.

In addition to Hedron Archive and Hedron Crawler, aother mana acceleration Artifacts you may want to consider is Seer's Lantern.

Paradox Haze is an awesome card. You currently have 8 cards with upkeep triggers, so I'm not sure if it works here. It's awesome when you do have those 8 cards, but when you don't, it's effectively a dead card. I'm not saying that you shouldn't use it, in fact it'd be awesome if you do use it, I'm just warning you of the possible down side.

I recently had success with Mutilate in my Korlash deck. -1/-1 per swamp you control to all creatures is actually pretty great, especially against creature heavy / dependent strategies.

Most people will suggest Painter's Servant to combo with Sphinx's Tutelage. While this is a good combo, I like the idea of combo'ing it with Teferi's Puzzle Box to increase the number of cards you draw each turn, thus making more triggers for Sphinx's Tutelage :P

or maybe Braid of Fire?

Just thinking now, Resonator makes the Honden from Kamigawa just a little better. I already use Paradox Haze with them, but Resonator can be pretty cool with them, and the mana required to play the Honden means that you should easily have {2} open in your upkeep (when the Honden trigger). In fact, the Honden deck I run requires so much mana that I run a set of Honor-Worn Shaku; which effectively allows me to tap the Honden for {C} each, easily supplying mana for the Resonator.

Searching for Bitter Ordeal is important. Dimir Machinations provides the same Transmute trick here. I also mentioned Beseech the Queen already, and there's always the classic Diabolic Tutor.

Maralen of the Mornsong could be useful too.

Also, Infernal Tutor can help fetch the second Myr Retriever.

Nice catch on Shadows of the Past. I didn't think of using the deaths beyond Falkenrath Noble. I would use Blood Artist, but Heartless Summoning kills it) and Pawn of Ulamog (for more creatures that die).

I like the 'All-In' route you've taken. I only have one suggestion: Elusive Spellfist

If you do stick with Modern, don't forget Transmute. Heartless Summoning and Myr Retriever cost 2, which allows for 3 cards with Transmute: Dimir Infiltrator; Muddle the Mixture; and Shred Memory (all have uses other than Transmute too). I'm looking to aquire a set of Beseech the Queen for the build I'm working on. Also some creature retrieveal (ala Disentomb), maybe Corpse Churn as it's an instant, to 'protect' the combo from graveyard hate. Needless to say, I'm trying for mono black.