Following the examples listed above, the Mindscape possibly provides those with access to it with a guide of sorts, who represents either a major part of that person, or foreshadows what they will become. Nick, Victor and Andrus all have guardians which are essentially manifestations of their powers. Dominic has Matthew, since they share a body now. Michael being the only one who hears Kalyx speaking could be a sign of this, possibly due to Michael's past service to Cepheus. Why this works for Sarah and Zero, I don't know, but recent events have shown that Zero is inside Sarah's mind unless called by Nick.
Void supplied the NUO with the Entropy General Monash uses in Dreadbay Prison.
As of the end of the original Ather City, we can safely assume that the only existing supply of Eden was in the hands of Void, and that there was no Entropy remaining in the city, or indeed anywhere else on the planet. Cut to a hundred and fifty years or so later, and Void has successfully re-engineered supplies of both Eden and Entropy from that one vial, and used the latter upon himself to obtain his fear aura ability. Another hundred years, and General Monash has been given a vial of Entropy by the NUO. If we're still assuming that the only supplies of either substance were in Void's control until recently, it's entirely possible that he might have convinced the NUO not to try and kill him in return for a sample of either or both of the substances, or otherwise convinced them in some way to buy samples of the substances for something else in the past hundred years of the planet's history.
This assumption is negated by the fact that it was directly stated that the NUO, while researching the ultimate destruction machine, took in a scientist who was, in fact, the son of the scientist who devised Matthew. Said son was also one of those who escaped Ather City, and thus, seeing that he implied having ways to access information from outside his shelter, he COULD have found out about Entropy. Anyway, he later reinvented or rediscovered (seeing that Entropy is extracted, not created) the powerful Mutagen. Hiding the information from the other NUO researchers, he kept working on it, till, at last, he discovered the black gooey Furor (self named), a distilled and processed form of Entropy with twice the powers, involving everything from 'any mass creation' to 'spontaneous mutation'. However, the moment he revealed the information about his discovering Entropy, he was executed and his Entropy samples were destroyed, as it could have led to universal mutations. However, his Furor/Paint supply remained intact.
It was stated that both Entropy and Eden had been extracted from Xill, who died long before the destruction of Ather City, and it was also stated that his blood specifically was then altered to provide the substances. It seems highly unlikely that anyone else would have blood anything like that of Xill's.
Xill was a mutant, and the Grey healer had specifically stated that he was altered to extract the mutagens. Xill was in no way different from other mutants, at least before his blood was altered. Considering that mutants were quite easily available for experimentation, the required substances could have been discovered after some trial and error.
It's not just mutation; Xill's blood type would almost certainly have had a part in how the chemicals functioned, and the mutations themselves would also have been a necessary factor in how they acted.
Another possibility is that Void provided the scientist's son with an early reengineering of Entropy and/or Eden, and they then worked on them separately from there.
Completely possible, though from Void's personality, it is difficult to believe. It could be that he was offered something in exchange, and, seeing that he liked to get intoxicated, he might even have been fooled into sparing with it. Anyway, as the NUO do not possess any samples of Eden so far, it can be assumed that Void died with the knowledge about the second chemical.
Point of fact: thanks to the way his digestive system (i.e. his stomach) works, it takes several bottles of vodka to get him drunk, and then only for a short while afterwards. That in itself would be rare, mostly due to how expensive such alcoholic drinks can be; thus, it would take a rather more subtle approach than that to simply fool him out of the substances.
It is possible that some of the Entropy samples remained, and Wallenquist later salvaged these in order to ensure a last ditch defense for important NUO centers. Furor, now referred to simply as Paint, is completely under Arthur's possession. He can create both Entropy and Paint by draining out existing humans.
In a city converted entirely into superheated, radioactive glass? I think not.
As for Void having used his vial of Entropy on himself, it is possible, but improbable. Void was never shown to be a chemical genius. However, it IS possible that he could have forced others to work under him.
It has already been stated that Void had been working on the vial of Eden for at least 150 years or so, gaining knowledge on how it acted as he did so. Re-engineering samples of Eden and Entropy from the original vial would have been fairly simple after a century and a half of practice; from there, it would have been simple for him to drink the vial, perhaps having done so in a situation that had put Void into a state of utter fear, based on how the aura works, and feeling that he had no other choice if he wanted to survive.
The entirety of TWA is a hallucination had by Dominic's soul.
After all, he was being tortured by the True King of Hell for five hundred (respective) years, and it has even been acknowledged that without Matthew's presence, Dominic would be a jibbering wreck. Also, it has not yet been explained how Matthew ended up at the Field of Black Glass at the exact moment that Dominic escaped... it seems reasonable that the whole scenario of the last two hundred and forty five years could have been dreamt up by Dominic's wandering, maddened soul as a daydream of sorts; for example, Void's death in-story and Dominic's self-imposed exile into the Mindscape could both be explained, using this theory, as extreme wish fulfillment on the latter's part.
The assumption that Void has any importance whatsoever to Dominic is completely invalid though, being little more than a nuisance and never anything else since the first game, so this theory has a hole. Aside from that, if it really was a hallucination then upon his exile it should've ended, since Dominic is more focused on creating his sanctuary than about the goings-on of the world outside.
They certainly didn't like one another in any way, shape or form. If Void was really 'just a nuisance', Dominic wouldn't have found it necessary to kill him. Again, wish fulfillment.
Michael's sanity has been slowly deteriorating as he comes in contact with members of the Old Guard and their descendants.
In the events of the first game, he goes from being ambitious and just flat out violent, then to paranoid and desperate to survive, actually making a scythe out of a snake demon's spine and a claw. Finally, he goes all the way to ordering Void to throw him at a Horseman of the Apocalypse and jumping out of a skyscraper onto a dragon and gouging it's eye out with his intact (only) arm while shouting, "All aboard the pain train motherf***ers." After the final battle of the first game, his descent into madness seems to slow, reverting back to violence. However, after the timeskip, he is shown talking to his sword (made from the remains of the above mentioned scythe), referring to it as the demon it is made from. Shortly thereafter, Void shows up. After being recruited by the Hive, thus meeting with Sarah, he travels to Australia, kills a giant spider, then eats part of it. He soon drives a makeshift dunebuggy, which has largely been stripped of it's tech, off of Ayer's Rock and grapples onto a steam/cyberpunk airship. He finds Connor when he's captured after killing countless guards, the punches through a solid pane of glass, pulls him through it, and then throws Connor and himself out of the airship over the Pacific Ocean, where he shows no regard as to whether they would die by hitting the water after falling a couple thousand feet. However, it has yet to be seen what the effect of being around both Victor and Void's grandson and granddaughter, respectively, will have on him.
The reason technology was wiped out by Dominic's sacrifice was because he was an electricity mutant
The closing of the portal to Hell wiped out electricity. That makes very little sense until you remember that Dominic was an electricity mutant. His sacrifice wiped out electricity because the True King decided to wipe all fragments of what he could control on Earth. That way, if he ever escaped somehow, he'd still be way out of his comfort zone because his mutation would count for nothing.
Not possible - TWA definitely has electricity in the world. It's just rather more expensive to obtain and use, is all, presumably due to more typical post-apocalyptic factors such as lack of workforce.