Remembering that I will not discuss again the doubts already answered. Only the new ones.
Resume:
It is very difficult to describe all the possibilities of Bastion, it is extremely versatile.
The Bastion and its Simulacrums have an Unbeatiable Stealth check. 36 ~ 46 + 1d4, magical guidance, advantage.. You roll once and remain hidden until revealed.
All the things he is proficient in, he is invincible. Stealth, Investigation, Perception, Deception etc. With Skill Empowerment spell + Trance of Order
With Subtle Spell, your spells are completely imperceptible. Since he is hidden and his spells are imperceptible, yes he can control any creature with Subtle Dominate Person, Subtle Planar Binding and simulate the creature's own voice in his mind (Telephatic and Actor feat.), with invincible deception check.
It is totally hidden against divination magic by RAI and RAW, because Nondetection is constantly cast. In addition to mentally controlling creatures mentally, impersonating (Actor feat) any creature and still inciting hatred against the Wizards, casting spells like Subtle Meteor Swarm that destroy entire cities and at the same time making people believe that the wizards are attacking. Your possibilities are innumerable here.
The limit is your creativity.
That manipulative power alone is enough to easily defeat entire schools of magic.
I call it
Master of Deception.
Twin Simulacrum: Yes, the bastion can have 2 simulacrums simultaneously.
How does he get these creatures?
Simple, Hidden and cast Subtle Dominate Person and will control any humanoid creature.
Orders failure of Flesh to Stone Saving Throw. He maintains several creatures that are statures, potentially his choice to be his Simulacrum.
About Assassin's Simulacrum: Is that DM fiat? Course is. But, bastion doesn't care about a specific Simulacrum, any strong humanoids can be targeted by it.
I'm considering assassin, because Bastion is a magical assassin and he's more likely to have access to assassins than other PCs.
So, Assassin level 20, Drow Mother, Cleric level 20 or any of these humanoids could potentially be your simulacrum (Like it or not.)
Understood?
Invincible in combat:
In addition to being imperceptible, versatile and hidden.
No Spells really work against Bastion.
It automatically passes all saving throw of wisdom, charisma and constitition.
With Trance of Order + High saving throw check.
Feeblemind:
He is also not afraid of Feeblemind, since his Simulacrum is always hidden, always. It can dispel it or cast greater restoration that simply nullifies the effect of feeblemind.
In addition, if Bastion reveals itself, its contingency spell is activated: Greater Invisibility, which makes Feeblemind unable to cast.
To cast Feeblemind, you need to see the target, with Greater Invisibility, he is immune against that effect.
In fact, it is highly unlikely that anyone would be able to cast this on him, he would be dead long before he realized it.
Feeblemind is totally useless against Bastion.
Mind Blank: Bastion may or may not have this effect, depending on the situation.
My level 9th spell slot, depends on the situation.
Right?
This completely nullifies Feeblemind or any effect that affects the mind.
if you think Feeblemind works, well, good luck.
Damage: Its damage is monstrous, with subtle empowered meteor swarm + Simulacrum's Subtle empowered Horrid Wilding. Since these effects do not comes from the caster diretion as fireball spell, they do not reveal their position while It remains hidden and imperceptible.
Control:
Its control is monstrous. Subtle Dispel simply nullifies any spellcaster defense, can dispel Simulacrums, can dispel Contingency or any magic effect. The best part is totally imperceptible. Subtle Polymorph is a death sentence + Subtle Flesh to Stone. There is no defense against that. Subtle Domina Person + Telephatic is amazing, Subtle Mass suggesion, Subtle Planar Binding. He can control any creature he wants and its effects are imperceptible.
Planar Binding:
It can be by Wish or by normal Planar Binding Spell. You are hidden, your spells are imperceptible, the enemy cannot be hostile to me, because he does not know who Bastion is, his effect is imperceptible and the command will be obeyed. This has been thoroughly explained in that thread.
About Demon Lords.
I already explained that The Bastion will dominate any creature in its path, no matter who it is.
Demon Lord or not.
Genies, Elementals, Angels, Fiends. Everything on your path.
Many spells slots:
Regenerating Sorcery Points:
With Sorcerous Recovery class featture, its CAP level 20 class feature, it regenerates 4 SP for short rest.
How many days without sleep. Well, Bastion has High con save, which allows him to spend several days without sleep and without falling into exhaustion.
This is not coffelocking, this is a fair use of your capstone.
Coffeelock is to use features like Greater Restoration, Warlock Invocation to not get exhausted and accumulate infinite spell slots.
It doesn't happen here.
Daily spells: Aid level 6 or 7, Armor of Agathys level 5th, Pass without Trace and Invisibility.
Mind Blank, depends of situation.
Skill Empowermen, depends of situation.
The Antimagic Field has already been explained, I will not repeat.