civilization
That's was a quick rating, I think you are right about the sheaim and you're probably in a better position to evaluate them than me since you started playing recently.
But I've found that the Scions are best used with lots of small cities to use each citizen to full extend. That require a lot more management but is worth it. And I put every civilisation that required more managing advanced. for example the mekara order can train and add slave to their city, each city can have one slave/4 population plus the bonus attribued by diverse building. Each slave add one sickness to the city and production/research/gold depending of the building. That alone would had been enough for me to rate them advanced.
For the doviello rating, I mistook them for the calabim. I blame erfworld
Research
But if you don't they won't understand they can get more research by exploiting tile with high commercial value.
If I got it right, you get commerce from the tile you exploit (and some building) and you distribute it between science, culture and gold as you see fit. Then the game give you your bonus gold from building and you pay your maintenance cost
Note that building can grant gold

gold

and/or commerce

commerce
Tower of Mastery
Never really tried to achieve a tower victory but that's how I'd do it. I don't think it would be too easy since you don't only need to get the correct mana type, you also need to build four 800

building and one 4000

to win. Abusing the great person spawn to get only great prophet and a fast altar victory is probably easier. I did it in 320 turn, using 4 mage with charm person as my only defense (I was in 3 war when I won) and hurried my last tower for 14000 gold
Gone to Hell:
possible, I've never seen a really high counter, even in long game with the sheaims.
Hamstalfar:
I think they're an hommage to Valkriion, the modder that made Rise from Erebus. One of their leader use the same name and forum image. I know the civilization didn't existed 3 years ago.
Grigori spawn mechanic:
The grigori have the best spawn mechanic. They have an adventurer counter (top left of the screen) that go up each time the great person counter go up (mainly from the specialist in city) when the counter reach 100 (+20 each time), they get an adventurer and the counter reset. Their farm and pasture can become homestead. Each homestead give one free stateman to their city. When the city have a citizen's forum (code of laws, writing) stateman will affect the adventurer counter. Their world spell also affect the adventurer counter.
For the horde work as intended, only the bhall units are affected.
I can't say why the cualli world spell give citizen instead of 4 production. Maybe the building was suppose to give 4 free specialist, maybe he wanted to add a way to make each citizen produce more (maybe there is, I wouldn't had cared enough to remember) or maybe it was to give them a specific feel. Hard to say since there's very little info explaining tit. The slave pit building lore insinuate the slave were once dead but it doesn't fit their starting mana (water, shadow and body)
I remember geting a underground slave market, upon disbanding it I got 2 free slave. So yes, "good" event for high crime rate exist. I'm not sure if the chain of event where you can put a mad scientist as headmaster of the mage guild is tied to the crime rate but you can get 2 world unit out of it (altough, the price is very high)
Not sure enough to answer your other question but some of them could be tested using the worldbuilder
idea: Make the cualli slave pit buillding give 3 specialist instead of 4 citizen. It would make a more fun gameplay and wouldn't affect the little lore we have about them
idea: To be more new-player friendly, add an option called "nerf barbarian" and "nerf them more" in the custom game list. Each giving a -20% strength promotion to every barbarian unit
Balance: The grigori sidequest ability (any unit in a stack with a fort commander) is probably supossed to use the affected unit move each turn. Currently the unit affected move before the player but the player can still move it afterward (if the units isn't in the fog of war) so he can simply bring it back to the fort each turn. It give 100xp to any of their unit in 50 turn.