In the current magic-system where you just need the sphere promotions to cast and not channeling (Priests having channeling is just an oversight i belive. Ganavon has been made able to teach deciples again for the very reason that what is possible again now shouldnt work at all.) it strongly seems like a feature.
Is a nice-power-up for having a lot of multiple mana sources of the same type. So i think that bit should stay. (and in fact its more natural / easier to understand and unrestrictive than the channeling-casting-restriction. The very aim of the merge.)
3 Nodes of the same type before researching Sorcery is not an everyday feat after all. (and Fire is not to be gained by wonders / shrines. If anything by the now not so safe Pyre and 2 additional Nodes + Elementalism. Or Palace + Pyre + Node or Palace +2 Nodes)
There is if i get it right no way to easiely get 3 mana of the same type without getting the corosponding Node-tech first. (ok for Shaim death is possible with Broken Sepulcher and Soul Forge from Malvolent Designs might be the exeption. But MD isn't an early game tech in its own right and the Soul Forge is neither cheap nor purely beneficial.). The Node-techs are one relatively small step from Sorcery i reckon. (And vasalizing isn't that early or easy either and usually requires some hard fighting before.)
Fire not even for the Amurites. Far less any other non-fire-starting-civ. So its their special spellcasting advantage, no going home for them enjoy it while that advantage lasts.
(On a side note: Luchirp get the very same advantage at the same tech level for the same price (Wood- or Mudgolems with blasting-workshops.) without having 3 nodes or a fitting palace-Mana. Still not everyone calls them the fireballing golems of death and its not a fool-proof strategy and is by no ways sending the Amurites home. Try them if you haven't allredy and like fireballing alot.)
To be honest i find the new Shaim with triple death at necormancy many times scarier. With 3 Death their Adept-summoned Spectres start at 6 Str! and invoke fear (with combat 5 adepts (Power 9! spectres anyone?) that reeks of Meteor-Shower! at adept level.
I already anticipate Ecofarm complaining.

Unlimited fear-inducing 3-turn lasting 9! sending everyone home.).
Oh, btw. their spectres grow stronger with any single node / death mana you can get / conquer with your spectres.

You won't want to fight those at adept level (and their now functioning diseased corpses are so strong that Kael felt they where to strong to be seen by us and had to nerf them. Still less than funny to fight those.).
But that fits their flavor allright and makes the Sheiam a bit more newcomer-friendly and a lot stronger in early to mid-game.
So im all for that new feature.
One of the best side effects to the magic system imo.
And this one Tier-2 Spell out of the Box at Adept-level doesnt allow you to learn or cast any other Tier 2 spells out of the box...)
So one or perhaps even 2 Spells castable at adept-level for owning a large land area is very much allright in my book.
And it helps the AI as well who is quite fond of building multiple nodes of the same type it allready has.
Even though im all for 2 Spells per Sphere at Tier 3 (Archmage-Level) and returning of unyielding order to sorcery sphre law 3 or 4 instead of order redundancy (social order and unyielding order) and double-whak impedement of late Game for all other Religions / non-religious civs. (the one spell change i passionately dislike the most. It has taken away alot of the late-game builder-fun. And by no means is stronger or comparable to Meteor shower since for a start you confine your Arch-Mages to your cities. And big cities hardly win a fight on their own.)