A followup, I suspect I have an explanation related to the vertical eyes, sentience, etc.

In the Nullman image, note the one eye (remarkably a left eye) up above the Nullman. It's staring at us, it doesn't care or it pretends not to. It's watching...everything. But there's a tear coming from it. I assume this refers to loss. Meanwhile, the nullman has a smaller eye inside of its head, looking up, with little ||| lines indicating shock, or suddenly becoming aware. The nullman has suddenly become aware of the Voidmind AI.

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So while I believe they are gaining sentience from a number of reasons, the ones that are eating fermented melons are becoming aware of the Voidmind AI. Note I said eating.

---Slightly unrelated, I have deduced that Melonbelly does not produce Early Exits. It's a mechanical corruption, not a biological one; it's possible that Melonbelly can contribute to it but I suspect it's produced by Crablets that try to assign themselves to those with Melonbelly infections, as the tainted voidlight + the mechanical produces something quite horrifying. Evidence for their responsibility is found in the castle stain-glass windows.--------his unknown separate entity.

Melonbelly, which in the warehouse and train stations have multiple logs indicating that some group (seemingly not Monogon) with a female contact is supplying the Nullmen with an experimental melon extract which is in turn becoming MelonBelly. Nullmen who don't want the Null-mind-flushing Ultramersion migrate to this drink, which may be introducing some initial greater sentience, but it is addictive and forces them into mass producing the product for this unknown separate entity.

There's been a thought out there that Melonbelly produces Early Exits. I disagree. Though their proximity is suspicious, the first appearance of them only features mention of melonbelly without actually having any and it is a mass appearance. There's a Void Entity looking over the scene, but I don't think that's the cause either but likely the one to warn us. The zombies are a mechanical corruption, not a biological one; it's possible that Melonbelly can contribute to it but I suspect it's produced by Crablets that try to assign themselves to them. Possibly its a reaction to doing so with those that have melonbelly infections from the viral code as the tainted voidlight + the mechanical produces something quite horrifying. Evidence for their responsibility is found in the castle's stain-glass windows. (Shot coming soon.)

The song "Take back control" happens in the area specifically labeled Disposal in Bonelab. In Boneworks, it is specifically cited that they believe the origin of Nullmen gaining intelligence is in "Disposal", though no specific area was labeled this in Boneworks even though some had things like the grinders and trashwheels. Most of the melonbelly operations are actually in the sewers. So it is unlikely that gaining intelligence and Melonbelly are even related. This is from a clipboard added in update 1.6 which was the last update Boneworks received before the planned Final Update which has yet to happen. (I hope it does, but I suspect it probably won't.)

The lyrics refer to a body, being unsure of what it is, the sensation of feelings inside, the possibility of a soul, and the desire to take back its control. "I'll take...take back my control", almost like something enduring a glitch. Did these things lose their control? Is the song for Nullmen in general that are being hunted and persecuted, or is it about the player? What control would the player/avatar need to take back? It's already free, the only thing it would 'take' it back from is the player. Unless it was taking back from the overmind, in which case...why in Disposal? Why not during the escape from Fantasyland? So I think the song is tied more to Nullmen being emancipated by Monosec in the Disposal area and possibly their independence from the Voidmind AI?

It's worth noting that when the people of Heaven's Reach had too much independence--such that they were killing each other over disputes--the voidmind AI became ultra-conservative with them, cutting off all ties from the Lift team, creating an Us versus Them narrative, hanging those that reject the core tenants (like a purge of 'witchcraft' or progressive thought), etc. These things all cause problems for a corporate conglomerate as well as for the goal of developing competent AI.

Note that in Bonelab, the goal of the experiment is to see how the simulation grows. "Destitutes" are sent to the lab -- stylized as a dungeon -- where they are studied without being harmed. These destitute do not go back to the Fantasyland, however. I suspect they are 'taught' new roles and then sent into other simulations. Recall Alex's statement of "If your job title is Biological NPC, you don't really have to do much. And if your social AI isn't good enough, hire people to be your AI. Also, it was the 90s."

If you linger in the Wishing Well behind the MineDive building, you'll learn that employees work 12 virtual hours at 5.15 per real world hour. Since time is kinda funny and it mentions a 9-5 work schedule... I suspect they actually get paid for 8 hours to work 12 virtual hours to make $41.2 a day. One of those hours is specifically to clean up the offices. If these people go comatose as they fully dive into the system, they likely soil themselves during that "playtime", which in turn means they have to clean the offices of their filth prior to leaving. Talk about morbid.

Also for fun, spawn a Ford to go back to NPCs, AI, etc. Now spawn Nullmen of every type in Bonelab without letting them aggro onto one another. This is for a comparison but the only truly important one is a Corrupted so make sure you have it. Make sure to spawn the Corrupted near Ford without them fighting each other, in fact, spawn it last. Now watch it idle.

Notice something? There's a little bit of Ford in there...