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I think you and I just have different interpretations of the setting lore at this point. I don't claim to be an expert, but from what I've read, I was under the impression that any Navigator sane enough to do their job was, by definition, a precious resource to be guarded closely. Particularly if they're only cosmetically mutated, as Kennoch is; he's still got the use of his limbs and everything. The Navis Primer paints an image of palaces, vast wealth, loyal staff, and substantial security. The "very minor" Absalom Dynasty you've described has a foothold on several planets and a fleet of city-sized ships. This is an organization with tens, maybe hundreds of thousands of direct members. An organization that goes to dangerous places, does dangerous things, and makes dangerous enemies with every chapter. I just plain don't think it's weird for one of the top-ranking members to have a whole five bodyguards. It makes sense that Anika chooses not to; she talks often about disliking other nobles and the systems that support them, and based on her mother's letter, I suspect she's always felt this way. I can see how a quirky archaeologist with esoteric interests would behave as she does. It took nearly burning alive for her to hire a single assistant. And hey, fair enough. She's a cool character and I like her--but she and Kennoch are from different backgrounds and have different methods.
Of course, the Doylist answer here is that I personally think minions are fun, the 40k setting seems (at least to me) to encourage them among characters with the level of importance and influence that ours have, the game system itself allows for them, and Destro has been willing to allow for this sort of play. I had the idea for a guild of voidborn assassin-bodyguards to go along with my character, Destro said it was alright, and so, I went with it. It's easy enough to get the minions out of the way when the plot demands it; the transport shuttles go down and we're scattered and on our own for a bit, or the tunnels we're exploring are so expansive that we have to split up into small groups, or an enemy challenges the command staff to a group duel, and so on. I also plan to have Kennoch recruit more in the future and build his own local branch of the guild as time goes on. It's just something I enjoy, even if they don't end up seeing much use outside of capturing renegades during the boarding actions. I mainly enjoy them for the fluff, because I think it makes sense that this character would have loyal guards of his own. *shrug*
Ironically, I figured that Kennoch only having one squad of guards would be suspicious to the rest of you, an early little sign that he's unpopular with the rest of his family. I assumed that, realistically, a Navigator would have more protection. Evidently that didn't come across as I intended.