Yeah, it'd be that one. 40k runs on Rule of Cool, the rules of its fictional universe are prone to change with the writer and it is, in the final analysis, written to serve as background for as many unpredictable stories as possible, some of which will invariably contradict each other -- and that's fine. They don't want to definitively nail down exactly how the Warp works except in the broadest of terms because they don't need to for the setting to be fun and all more precision would do is touch off more tedious lore pedantry and make people's armies and fluff and so forth contradict canon.
If you want explosions and action, 40k does it on a grand and frankly goofy scale. If you want grim dark war stories, there's more than you can shake a stick at and some of them are actually pretty good. If you want to shoot daemonic photocopiers before they can corrupt more Munitorum forms, it turns out there's already a story that's indirectly about possessed office equipment, and as a Dark Heresy campaign I can see several ways to have fun with it.
If, however, you want the setting to somehow acknowledge or reward that you can qualitatively describe the broad strokes of how a photocopier works, I'm afraid there will be a lot in this setting that will upset you.
Yes, no, kind of. There are some ground rules WH40k tends to follow, and while there are exceptions to it, it's kinda understandable considering the scope and how many writers have contributed. Warp is very much the embodiment of "no real rules", but it follows at least one pretty closely that tends to explain a lot - whatever people believe is true, can be true. People don't really question nightmares when a bed or photocopier starts to eat them. The other one is tied to that Materium tends to not give a shit about what people think, and unless Immaterium becomes more "dominant" in some space, stuff will still follow rules of nature as they are. Immaterium becomes less dominant the more "extreme" some space starts to be, it usually involves direct exposure to warp (which is why ships with broken Gellar fields tend to be "fun" even if they come back to real space), someone doing some psyker shit or some really strong feelings happening in the place in question.
3) The void demons ARE able to permanently inhabit otherwise inanimate matter and change its behavior permanently, but doing so comes at some kind of intrinsic and permanent cost to them, such that the option of "inhabit all matter in the galaxy at the same time" is not on the table for them. However, they dont seem to be suffering in any way from their rampant infestation of inanimate matter all over the damn place.
Daemons can, in fact, inhabit Materium and directly impact reality. The problem is that Materium, well, tries to follow laws of nature, which are pretty much roughly as we know them - it requires weakening the reality for a daemon to be able to materialize or possess something, which is why you don't see daemons commonly, unless it's in Warp itself, Eye of Terror (or other assorted Warp storm) or someone specifically did a ritual of summoning or possession. Most of Galaxy still retains it's grasp on reality.
1) If the void demons can actually bind enough immaterium energy to the physical plane to permanently alter the behavior of matter-energy systems (like a computer, which operates the way it does due to quantum electrodynamics), then there is no way imaginable for the forces of order to ever win. The very air you breath, the water you drink, the food you eat, and the bed you sleep on all can be infused with so much immaterium that all of them literally murderfuck you to death, and then resurrect your corpse with a demon inside, with no recourse or means to prevent it.
Setting aside summoning the material forms of daemons, and focusing on possession, possessing something as basic as air, water or other similarly basic things is not really a thing - possession is intrinsically tied to the psychic link the thing in question can have with warp - in other words, it's soul - which is why beings without souls cannot really get possessed (directly, at least). Air, water, earth or beds don't really have any "soul" inherently, HOWEVER this doesn't mean they cannot have one - Warp works essentially in make-believe fashion, so if someone believes their bed has feelings strong and long enough, it might just be that the bed starts to develop some form of consciousness. Think of it as
this, to some degree (but without actual real-world explanations because warp doesn't like real-world explanations). Warp is quite literally magic.
Now, things that have some actual capabilities for basic thinking, such as computers, develop souls easier, since there is less "make-believe" going on, and more reality. This, in addition to the fact some of Imperium's computers are quite literally some poor sod's brain-in-a-jar serving as a computer because it's cheaper (and brain-in-jars already have souls) and the actual AI's that Imperium does still use, even if unwittingly, is what constitutes Machine Spirit. Turns out as much as people make fun of Mechanicus for being a stupid cargo cult, they are actually onto something.
So, if a thing has a soul, it can have that soul's connection with Warp hijacked, and thus be possessed by a daemon. However, it should be noted that daemons aren't particularly hyped about being toasters or beds (or a pile of sand, for that matter) - they like if their bodies are more capable, especially in murdering department, which is why they usually possess humans or really powerful machines of war such as Knights, not to mention a daemon can have trouble actually leaving the thing it's possessing unless it "dies", which, for say, a pile of sand, is problematic. Since again, it's very costly and hard for daemons to possess things, that means your bed is probably not going to be possessed anytime soon, even if it develops consciousness. That said, daemons can be forced into possessing things, which is often done to enhance capabilities of weapons and such, however the daemon inside will probably hate the one who forced it, and will try it's best to escape and possibly somehow punish the person, even if it comes in few hundred years, since daemons hold grudges pretty well.
2) If they instead only do so long enough to reprogram the AI inside such computers, then one could simply cut AI, and computers strong enough to house AI, out of the equation completely. This seems to be the angle the Imperium has taken, with banning AI altogether, and requiring any system that processes data to require a human mind be a necessary and functional part of its operation. This implies that it should be possible to completely wipe a contaminated system by restoring it to factory defaults, but GW wants to play it both ways... which makes no sense at all.
Possibly, possibly not. It might be so that possession has physically changed the system, so restoring doesn't work anymore, also daemonic taint tends to remain despite there being no daemons in sight anymore - this can serve as one of ways to weaken the reality, which obviously makes subsequent daemonic possessions or assorted manifestations (even something as basic as simple whispering of insane ideas can get a man to fall after a while). Also, it should be noted that direct daemonic possession is just one of multiple ways a computer can be made to start randomly murdering and in general acting very chaos-like. Scrap Code is essentially Chaos computer viruses, which corrupt Machine Spirits directly, or simply make simpler mechanisms (such as toasters) do something harmful, like explode. There is also actual old-school rogue AI's that aren't necessarily Chaotic in nature (however, some of them willingly strike a deal with Chaos Gods, in a manner similar to how biological beings fall to Chaos), just want to kill humans for the old good fun of it, and probably bunch of other things, including just regular non-Chaotic fuck-ups.
Until I have this issue resolved in my head, I really have a hard time with the whole "It got possessed!" angle. In my mind, I see "But it's a photocopier. It's all analog circuitry with a high voltage corona wire that attracts electrostatic dust, and then fuses it to paper with a big flash lamp inside a metal roller. Where the fuck is the ghost living inside this thing, and if it can, what stops it from inhabiting say-- The officer's mess, and getting into the whole damn bridge crew that way? Because inanimate matter is inanimate for a goddamn reason you know."
Officer's mess is bigger, for one. Photocopier probably broke a lot and people got mad at it, and kicked it, and it started developing feelings and hatred towards humans. The photocopier is just an easier avenue of attack due to those emotions, though it should be noted it's an incredibly hard anyway, a human would probably be much easier. This doesn't of course mean the Officer's Mess can't be Chaotically tainted and getting the entire bridge crew, it's possible, but it's less of a possession by actual daemon and just the place having weakened connection with reality which allows various Warp shit to leak in. At best, you'll hear whispers, see things in the corner of your eye, maybe some ghosts, at worst daemons will start materializing and coming out of the walls, but the room itself will still not be possessed in strict sense of a daemon inhabiting it as an object.
Once you go there though, it becomes case 1, and the universe is doomed.
No to doomed Universe (for one, it'd just be Galaxy, as we have no idea what's going on in other galaxies, and since there is no real psychical link with other galaxies, there is no Warp-based link to them). As I said, for daemons to possess something as hard to possess as photocopier or a toaster (or a whole room, for that matter) that place needs to be already pretty fucked, so you'd be rather worrying about actual daemons physically popping up to tear you in half rather than the photocopier making scary images.