Fair warning, this might be a bit long, and/or a number of points.

So, Daemons and Chaos are among the most terrifying enemies the Imperium faces; the very Emperor took steps, failures though they be, to curb the influence of the Ruinous Powers, and even as the people are forbidden to know daemons are real, they know that, out in the warp, and the dark, there are unspeakable, unfathomable nightmares, truly out for their very souls, but what can a person with the resources, and the connections, of a Rogue Trader do about them?

I assume that they know the signs of the Chaos Gods; you see this logo on a cruiser, you know Nurgle's faithful are coming to bring you some love, and you can act accordingly, but do they know what's really on board, assuming they aren't heretics, themselves? Can RTs know about daemons, or only Inquisitors, and the Grey Knights? As consummate shapeshifters, perfectly able to look human, it might be good to know something dangerous is out there, and it won't look like you think.

I know it takes a ritual to get here, but how long can a daemon remain? I know there is one emissary of Nurgle who looks like a lazy pile, in a heavy vac suit, but he's certainly a daemon. He might be surrounded by diseases, which might sustain him, but how might the other daemons last, without doing their obvious vice, and giving themselves away?

Can you repel a daemon, without being a psyker? Reveal one? What are good tools to fight them, once revealed? Always thought it was weird that reatures of the warp couldn't foul your attempts to kill them with it.

Last one, and slightly unrelated. Do you NEED to be an Ecclesiarchy envoy to bear a rosarius? I like them, and they sometimes have a better field rating than a refractor field, but without the flash your allies might need goggles for. If they are ONLY available to clergy, though... I have an NPC who wanted to ask the Emperor to protect her daughter, as she went to take up her warrant, and she knew that "the Emperor's light, from the rosarius, seemed able to ward off harm." (I assume she also knew about the field generator, but these are people with faith in thee God-Emperor.) Could she get a rosarius, maybe through a trusted cleric, and give it to her daughter, the new RT? Would other people flip, knowing she's NOT a missionary?

In story, all of this BS is me wondering how easily my womanizing RT might fall to Slaanesh's tempters. As Qel-Drake picks up new prospects, could anyone have a chance of catching one? Could they prevent one from being around? Drive it away? How to fight it, once they learn of it? How subtle could it be, before having to do ridiculously Slaaneshi things, and making people suspicious, if it can't get the man, right away? The Rosarius bit is for someone else in the fluff, but I don't know if, aside from being easier for the missionary to get, if ONLY they can have them?

"My lord I'm afraid your latest conquest has left you with a case of Slaanesh influence. The infirmary isn't equipped for this we'll need to get you to the temple of the God Emperor."

Slaanesh is one of the trickiest, though if done right most rewarding Chaos Gods to focus on if you are going for the corruption of a once proud character, or their strength of will in resisting his charms. The easy way out is devolving the game into literal pornography, and with the right (or wrong) group this can work... for a laugh once in a while. But Slaanesh represents so, so much more than that. The temptations of the Prince of Chaos can take the form of half-heard whispers during political conversations that seem to give the exact response needed to swing the debate, or a seemingly miraculous change in the fortunes of one Endeavour that results in a long-treasured item finally being acquired. Of course the trick is subtly weaving in hints of unforseen costs to such events, making the player doubt his own actions and eventually, if he catches the hint, the integrity of the character's very soul.

As for Daemons in general, while psykers are your best (and worst) deterrent to the predations of the warp, any sufficiently trained individual has the ability to see the effects of Chaos or at the very least hints of potential daemonic activity. This can take the form of looking for errant behavior in crew-members, changes in personal patterns that could hint to a daemon's influence or at worst a possession. Once found though, the presence of a daemon can be repelled as you say even by non-daemons. The most famous methods of course are the Ecclesiarchy's much vaunted exorcisms and rituals of warding or banishment. Holy oils, blessed parchment with prayers of devotion and hatred, hexagrammatic wards, all of these things represent ways to ward off daemonic creatures or corrupted individuals. However, it is an ironic truth in 40k that the weapons and tools of the enemy are at times equally potent against it. Daemonically infused weaponry, cursed artefacts, unholy unguents made through blasphemous ritual, these can at times be used quite effectively against the very thing they were made to serve.

Finally as for your question on the Rosarius, I've always run it as something that represents the blessing of the church upon an individual. It marks you out as a chosen among the faithful and someone the common folk can look up to as one blessed by the God Emperor. Does this make you an immediate member of the church? No, but it would open many doors within that monolithic institution for those looking to profit from it.

Space Marines are all about killing things in the name of the Emperor, but they're not constantly falling to Khorne (just periodically), because there's more to falling to Khorne than just killing. Similarly there's more to falling to Slaanesh than having a different woman every night of your life. Falling to Chaos is the moment when you start killing, hedonising, scheming or... plaguing?... not for a goal or for any other reason, but simply for its own sake. If you lead a well-balanced life then there are fewer hooks to instantly falling to Chaos, although you certainly put yourself in a more imperiled state than other people.

I view most people in the Imperium know that Daemons exist and can take many forms, but not perhaps much else than that. However this is why people in the Imperium aren't encouraged to be curious. If you're surrounded by someone who's dressed in a corroded void suit with flies buzzing around them and a sunken, barely visible skull inside you just shout "MUTANT" and burn them with a flamer. If you're curious (and survive) you might grow a discerning eye for these things. However that eye might end up on the back of your neck or something.

Regarding Rosarius's (and psychic warding), I do restrict those unless my players have an in with certain members of the Ministorum, Inquisition or an area I find appropriate. You might also allow them to acquire one, but then they've actually just bought a normal force field inside of a rosarius shell. See how long before your players figure it out!

Space Marines are all about killing things in the name of the Emperor, but they're not constantly falling to Khorne (just periodically), because there's more to falling to Khorne than just killing. Similarly there's more to falling to Slaanesh than having a different woman every night of your life. Falling to Chaos is the moment when you start killing, hedonising, scheming or... plaguing?... not for a goal or for any other reason, but simply for its own sake. If you lead a well-balanced life then there are fewer hooks to instantly falling to Chaos, although you certainly put yourself in a more imperiled state than other people.

I view most people in the Imperium know that Daemons exist and can take many forms, but not perhaps much else than that. However this is why people in the Imperium aren't encouraged to be curious. If you're surrounded by someone who's dressed in a corroded void suit with flies buzzing around them and a sunken, barely visible skull inside you just shout "MUTANT" and burn them with a flamer. If you're curious (and survive) you might grow a discerning eye for these things. However that eye might end up on the back of your neck or something.

Regarding Rosarius's (and psychic warding), I do restrict those unless my players have an in with certain members of the Ministorum, Inquisition or an area I find appropriate. You might also allow them to acquire one, but then they've actually just bought a normal force field inside of a rosarius shell. See how long before your players figure it out!

Yeah, I can see that. I might argue that plenty of those same Space Marines DID fall to Chaos, even if many of them had little more reason than "my Primarch, who might've only been back for weeks, did it, and I just followed him", but then I'm just being an ass. Just was unsure how much of an interest some daemons might take in some people, with the galaxy's size, and what they might do to claim said soul, along with how someone who is as questionable, and grimdark, as the setting expects resists these terrors. When daemons are consummate shapchangers, it seems about as easy as it would be for a Genestealer Hybrid, with some lax medicae. The man in question is far to ridiculous, and silly, to probably fall to Chaos, but Chaos might still try?

I know that, in some writing, and in other verses, someone might go to the Church, in this case the Ecclesiarchy, and ask them to bless the endeavor, in the name of (the) God (-Emperor). if the Ecclesiarchy can bless weapons and armor, maybe even bestow some cheesy sword/spear/artificer suit/cloak of legend (the Explorers MIGHT even be able to take Piety's Charge, from Everharvest, or something), I didn't know if they might bequeath a rosarius upon the leader of said expedition, in the hopes that the GE's protection would follow them, perhaps the dynasty's chief priest gives the same ne to each ruling scion, or something, or if they would say "well, blessed weapons, and sanctified armor are one thing, but a rosarius is ONLY for a priest, and then only one of high rank." I'm sure on some level, I'm also just being whiny, because the lower field ratings are lower, and the higher ones are all xenos, AND impossible to get, while several are only a small coverage, or with batteries worse than their Star Wars equivalents used to be. Mostly, I just wanted something with the rating comparable to a Conversion Field that didn't necessitate goggles, or murdering an Autarch, and while the Rosarius IS a smidge shy of the "4+ Invul save" it could've had, it's so close as to fit with the percentile system this game uses; I just didn't want it to be a sin with the Ecclesiarchy to have one. The woman in case might not be the most pious, but her mother was thinking piously, and probably wouldn't have done so if it was a (religious) crime. Again, thanks Erathia.