Dear dev team:
It's that relative newcomer again. Thusfar, I've only played 3 full games, 2 as Amurite and one as Sheaim. Despite my inexperience, I hope we can at least talk about the following:
Summoner is more than just overpowered. It is massively, amazingly super overpowered. +2 turns, as you pointed out, could result in that theoretical massive stack of 24 powerful summons from just 8 guys. That's already more than overpowered. Every AI city is ruins by the 14th summon, at most.
To then act like making it a stack of 16 units (+2 turns to +1 turns) WHILE giving them all a substantial 20% boost makes it all better sounds like grounding your kid for cursing but sending to his room with his TV and computer (with FFH2 on it).
I recommend the following be considered:
Summoner does the following:
All summoned units last for +2 turns.
BUT
Each Caster can only summon ONE of those units at a time. Just like skeletons. Except they're skeletons which die after 3 turns.
To me, this seem like the very definition of "reasonable." Still a very nice trait, but no where near a 2 million pound gorilla anymore.
The fact is, in my experience, having 24 maintenance-free powerful units is beyond unreasonable, and 16 is still ridiculous. With the above change, 4 archmages = 4 Fire Elementals, end of story. These semi-persistent nuits are ALREADY sufficiently buffed by virtue of the fact that they're semi-persistent. 3 turns is plenty of time for manuveuring, buffing, a few promotions, and multiple attacks.
Alternatively, if rent-a-hordes are absolutely unremovable features of this mod, I recommend crippling these units in a manner directly proportional to the number of extra turns you want, ie:
Summoner:
All summoned units last for +2 turns
All summoned units have -20% power.
Summmon spells can be stacked.
Or
All Summoned units last for +1 turns
All summoned units have -10% power.
etc
I'm not a modder, but as a writer or even back when playing with legos as a kid, I know the sharp pain of forcing my self to reduce and chop down my creations, but some things are just so overpowered they can't be fixed without tough crackdowns. This "ok, they only last TWO turns now, but they get extra power! And you can still have a indomitable, maintenance-free horder within two turns!" is just... nothing. If it's still a indefatigable horde, nothing's changed.
I'm sorry if that came off as harsh or amateurish. I just want to steer the discussion towards more radical yet simple solutions. At the very least, tell me I'm very, very wrong and why. I want to know. Because my game as the Sheaim was a cakewalk.
I never even MADE it to teir three spells. A stack of 16-20 Wraiths for the maintenance cost of a few mages was so ridiculous I was able to eventually split my army into 4 stacks (which would become megastacks at the drop of a dime), something I've never been able to do before. And this was with practically nothing of my economy devoted to my war machine!
When playing more games you will realize that in FFH2 Summoner is in fact not overpowered in comparison to other civs (neither has it been before the change. It was only Keelyn which was really over the top. Also the summons are in fact not maintenance free. Or at least not all of them. This is very easily seen when summoning many units and)
What has distorted your evaluation here is that
a) you still seem heavily influenced by vanilla civ / bts or other games that value balance over gameplay and / or are balanced primary for competitive multiplayer (where utmost possible balance is mandatory to guarantee at least the possibility of a solid experience.).
b) In FFH 2 most civs are "overpowered" by vanilla civ/bts standards and the civs are balanced by making all civs so overpowered that it evens out.
And one of the most powerful units in the game are arguably plain simple warriors massed for a rush right at the start of the game. If the sheaim (player or AI) will be targeted by that one chances are they won't reach even Knowledge of the Aether, let alone the point where they can reasonably leverage their trait.
Archmages / Eaters of Dreams might seem! very overpowered but they fill an important role: A tool to wrap up game instead of it turning into a stalemate. So that extreme power is actually intended to close the long game with a bang not a long tiresome and boring grind (most high-tier4 Units act like that one).
c) also the balance is done mostly at macro-level so you can't compare the power of Sheaim summons purely to the power of summoners from other civs without the summoning trait since its the Sheaim strong point.
If anything you would have to compare the Sheaim as a whole for example to the Svartalfar (which strongly favor recon and an extremely strong economy) or the Hippus (which strongly favor the mounted line and warfare overall) as a whole and you'll see that the Sheaim are not all over the stratosphere (and the way you spoke here very much indicates that you think the sheaim are beyond anything that is reasonable) in comparison to them or in fact most other civs (since the majority of civs here is strong to very strong. The common or weak ones are actually the minority.).
Some might argue, that those 2 are in fact quite a bit stronger than the Sheaim despite their extreme summoning powers..
That still doesn't work out perfectly in all cases (like Amurites vs. Sheaim). But it simply needn't be. Since the focus of design for this game (as per the team's comments are replayability, diversity and to a much lesser degree a good singleplayer experience. So balance isn't even the main focus.
You can't please everyone's taste equally and some things that cater to some players diminish the fun for others. There is sometimes no way around it (here in singleplayer experience vs. multiplayer experience.).
If you are looking for perfect balance as a primary decider if you have fun with a game (Whatever reason there might be for it. Since there are many sensible ones...

) you better look elsewhere or at least don't let yourselves be deceived by the illusion that it would be a primary goal and will ever be aceived.
d) especially in your case, since the Amurites are one of the rather weak civs and the Sheaim are arguably one of the most powerful late-game civs and surely a powerful one overall also in Multiplayer. (also you seem to have missed that while the Sheaim can field 8-9 Archmage-power casters which can summon 24+ Tier 3 Summons the Amurites can actually field 17-18 Archmage-power casters which can summon 17-18 Tier 3 Summons each turn or other spells or 25-27 Tier 3 Summons in an emergency thanks to their Archmages and Liches having Spellstaff for free (all Units Mage and up in fact.). All with more powerfull damage-spells and other arcane advantages, Have far easier acess to a much larger number or Adept-Level and especially Mage-Level summoners.
Still the Sheaim are stronger in their niche thanks to some aspects. But shouldn't they be? Its their point of power after all.)
Summoner actually is an average trait overall (because it comes into play rather late, many a player has finished a game prior to even the time most get to research sorcery at...) when coupled with their civs/Leaders (Sheaim and Keelyn only).
Which are inseparable since it can't even be chosen by adaptive or gained by insane i believe so here the civ-balance term is fully acurate.
The problem here really has only been puppets and Keelyn of the Balseraphs (which could get a whopping 72! summons from those 8 mages you mentioned or just needed 3 to field an impressive 27 Summons. Hence some of the participants in this thread said that the nerf should target only puppets of Keelyn. Not everyone who has summoner... Since Keelyn is still ahead by far. Still having the potential to summon a whopping 20 Summons with 5 Mages / Archmages).
Your suggestions whould make summoner downright awful, and the reduce the Sheaim only to pyre Zombies, which are their only real perk in terms of raw power when compared to other civs.