CMT

Sphinx's Tutelage is a much better replacement for Curse of the Bloody Tome: same CMC, better results

I know, but I am actually trying to build this deck without spending a fortune (I have it online exactly as you see it here, but not yet physically) so a few cards have been left out of the list because they cost more than I am willing to spend on a single card right now. That includes Wurmcoil Engine, for instance. And also, Ulamog does the whole job most of the times, the rest is supporting cast ^_^

Elusive Spellfist can be killed, though. The Nettle too, of course, and so the Nursery, but an enchantment is usually harder tp destroy than a creature, while killing Spellfist & Nettle with a single spell wouldn't be too hard.

Maybe replace the Relic instead. Ornithopter is still a blocker in case of need, Relic does nothing. Sure, Relic cannot be destroyed, but you're not really afraid of it being destroyed, because you can return it to your hand in response (unless you played it without setting the combo first but... why would you?)
The Phantasm is needed to search for the Nettle and/or Nursery (which is needed for the deck to work since it's where the infinite damage comes from, unless there are other cards that do damage when a colorless spell is cast... and anyway, 3 is too much?)

Well, yes, a land would trigger the altar once, it doesn't even need to be an artifact land for that, but that doesn't have much to do with Jeskai Ascendancy, does it? :-?

Actually, that's NOT the point :-) Even if it worked on lands, playing a land does not trigger any of the effects, Lands are not spells, and both the Nettle and the Nursery are triggered by casting colorless spells.
Also, even if you could send the land back to hand, you could play it again once (at best).

Uhm? How? Jeskai Ascendancy would not help untapping the Nettle (meaning that it would already untap by itself without it), and an artifact land wouldn't actually trigger anything

Maybe I am mistaken, but as far as I know Ancient Stirring can only find something as long as it is among the first 5 cards on top of your deck. And you have to hope that you don't have ALL the cards you need in those 5, because you are putting 4 on the bottom of your deck in that moment and regret it for the rest of the game ^_^; I'd rather use Ponder instead, you see less cards, but you don't actually have to put any of them out of reach, and you don't need a third color. Or even Taigam's Scheming (no draw, but you can order those 5 cards the way you want and get rid of any you really don't need, still without a third color).
As for hybridation with duskmantle mage/mindcranck... I don't know, the nettle might be a good trigger for the combo, but you'd have to splash red just for that when there are so many others that work well with BU. Aside from that, I wouldn't know how to merge theme effectively

We sort of commented simultaneously :-D

With my luck, the very first card on top of my deck as I cast Primal Surge would be another Primal Surge. With other people's luck... well, maybe you still need quite a bit of luck to have the correct cards in your opening hand to actually do all this by turn 3, so what's plan B? ^__^;

The deck is nice, a concept I like by the way, as it is somehow similar to the Duskmantle Guildmage+Mindcranck combo that I still play. Compared to it, it's more mana-effective (once you have the cards in place you only need one blue mana to start the combo, and you could technically play the whole game with 1 Island, 1 Mountain and 1 generic land and still be fine), but probably harder to pull. You need a 5/6-card combo (nettle + 0-mana artifact +helix or knack + altar and/or nursery), the combo can be stopped by either killing the Nettle or destroying the Altar/Nursery (which is why you should have both, so losing one doesn't matter even mid-combo) and there aren't many counters to prevent this. Also, you can transmute for the nettle or nursery, but there is nothing to help you find the altar and the Thopter/Relic. And you can't play the Nettle and start the combo in the same turn, so you need to protect it well.
Very good budget-wise, you could replace Serum Visions with Anticipate (needs more mana, but at least you have a saying in which card you draw, instead of drawing first and scrying later) and Cascade Bluffs with another mana source and get the whole deck for under $20 probably.

Either you tap the nettle to do damage OR to send the Ornithopter back to your hand. If you don't send the Ornithopter back, you cannot cast it again and untap the Nettle. So no, the Nettle does not kill. :-)

I know Caves of Koilos are faster, but in this deck Scoured Barrens is actually better as it triggers the Bloodbond Vampires and the Serene Stewards. I would drop an expedition envoy in favour of a fourth cutthroat, especially since you have Bone Splinter and Altar Reap to exploit the death of your creatures for added bonus, so you definitely want to always have at least one Cutthroat on the board. (Honestly, I am not too fond on Bone Spliters for this kind of deck. Altar Reap can be used to kill a creature that is going to die anyway, and draw cards as an extra, but Bone Splinters is a sorcery so it's just to exchange creatures. Might be still good, but I'd rather do that with more speed)

I suppose you have the Blossoming Sands and Nomad Outpost only to maximize the effects of Unified Front. Makes sense, but it may also never work out, and so many lands that enter tapped (4 of which only contribute white mana anyway, except for that card) might slow you down, which is not what an Aggro deck needs. I experimented with Unified Front myself and found out that getting a few more allies at sorcey speed didn't really help much.
I would also replace Healing Hands with Tandem Tactics. It doesn't make you draw, that's true, but it works well in combat, triggers your healing effects anyway, and can be used to turn the Nirkana Assassin into a 3/5 deathtouch creature all of a sudden, as well as protecting another creature of yours.

Consider adding one or two Fascination. It can be a game finisher in a Sphinx's Tutelage deck

I have a similar deck myself and I found out pretty soon that it was going nowhere without removals, so I added Harsh Sustenance (which can become a pretty powerful spell with so many creatures around) and Merciless Executioner. I also ditched the Disowned Ancestor and added Mardu Strike Leader. Battle Brawler is also very effective in a BW deck.

I'd suggest replacing Retreat to Hagra with Retreat to Emeria. The former can give you 1 life drain, the latter can put a 1/1 Ally on the board, which not only provides you with an extra creature but could potentially trigger 1-4 Kalastria Healers, thus providing the same effect as Retreat to Hagra but multiplied.

You should replace the guildgates with Scoured Barrens, same result plus a little life gain which is never a bad thing

Exiled creatures don't trigger "die" effects as they never reach the graveyard