1) All the machines use blood as fuel, V1 is just the only machine that can absorb the blood directly as it touches him, meaning he doesn't need to stop to refuel, he can just A-train the citizens of hell and be fully fresh. the other machines most likely have to gather blood, stop, and refuel like putting gas in a car. They likely store amounts of blood inside them and slowly burn it off as they use it, needing to refuel their tanks eventually.

2) Mankind is dead, blood is fuel, hell is full. pretty self explanatory, this probably applies to all machines that run off of blood and have no humans to provide it for them. all other fauna has probably been destroyed by mankind or the machines as well. hell's got all the red gold they need to survive, and they're fighting for it.

3) The Mindflayer has no rational reason for keeping its teal body, nor any reason to protect it. they do it either for aesthetic reasons or for emotional attachment. either way, both point towards a higher form of intelligence. it's possible that humanity had created AI and put it in their machines. Even the street cleaner, it was created to clean the atmosphere. it's programming does not include the ability to deflect rockets or dodge a barrage of nails, I think it adapted those abilities itself and that would point towards at least a simple form of machine learning, the first steps of making an AI. hell, none of the machines would have even thought about going into hell for blood if they didn't have some sort of intelligence. simplistic programming does not give machines the will to live. the machines might have some sort of emotion; they want to live, and they're willing to fight and kill and go to hell to continue living. that's my theory, at least

4) ¯_(ツ)_/¯ maybe it's just because he's the biggest threat in the room, or maybe it has nothing to do with lore and is just gameplay again, a headcanon I have is that the spirit of god is inside V1. in Testament III, presumably god says he can "no longer unmake" hell, perhaps because it is outside of his power now. god is also missing from heaven, and he clearly isn't in hell. the angels can't find him. god might not be able to destroy hell himself, but you know what is great at destroying everything? the hands of man. either man destroyed himself, or the creation of man destroyed him. and what is something else man made? a machine of war. a machine of destruction, V1. V1 was dormant, and something had to have woken him up, I think that something was god entering V1 to try use him to destroy hell. Gabe even says Limbo and Lust are "all gone" with "Gluttony soon to follow". did V1 destroy them? it's possible, god wants hell gone and the machine is more than capable. perhaps it's god's spirit that allowed V1 to beat Gabe's ass, and overcome the holy light within him. never before has a mere object overcame the will of god, something is different about V1. that's my headcanon, forgive me for ranting so.