I see where you're coming from, but I still don't think you could lump Warp and Daemonology (for example) together.
Say you're a Void Born, living on ships your whole life. You know a thing or two about the warp, how it works and how it can affect people. This doesn't qualify you to start creating wards against daemons (which can be done with Daemonology Lore at +10).
Similarly, you may be an Ordo Malleus Adept who is learned in matters of daemonic lore, but has never travelled offworld. You've read your Liber Chaotica and several heretical accounts of summoning rituals and the like. You could set down wards to prevent daemonic incursion, but if called upon to gauge how long a journey through the warp might take, or the effects a prolonged journey might have on crewmen, you wouldn't have a clue.
Hopefully that makes sense. I think it could similarly be applied to Psykers. Being able to judge the power level and disciplines possessed by a witch does not automatically qualify you to gauge the potential corrupting/daemonic effects a mysterious, interdimensional portal may threaten.
I'm not saying there isn't overlap, or that a character wouldn't have several of these lores. What I would say, though, is that a character may lack certain key knowledge that would be covered by an all-encompassing Lore incorporating Warp, Psykers and Daemonology whilst still qualifying for other parts of it.
Void Born don't start with Forbidden Lore: Warp. Even spending their wholes lives doesen't mean they actually KNOW much of the Warp. They do treat Navigation: Stellar as a basic skill though.
Forbidden lore is so for a reason, it is maddening, corrupting and lets man know what he should not. How long it may take to travel through it is not one of those.
Now summoning Daemons will require forbidden knowledge for sure - the more specific the better but it is clear Daemons comes from the Warp. Knowing as much as you can about it will help (but even so you need knowledge of specific rituals to actually summon anything).
Forbidden Lore: Psykers could still be part of a single skill I think, even though it's not always clear what this skill entails. Maybe it's the specifics on how Psykers are chosen and become sanctioned, maybe it's about the different power levels, maybe it's the fact that psykers are an evolution of mankind. Still it is so narrow a subject that it can easily be combined with a skill that has knowledge of the common denominator here: The Warp.
And yes, the character may indeed lack knowledge of these specifics. That is why there is a test. Say you have 35 int (pretty smart) and Forbidden Lore: Warp. You ask the GM if you know the basic theory behind a Gellar Field, it might be a Routine (+20) test. However, knowing that a mad psyker is infected by Psychneuein after studying him might be a hard (-20) Forbidden Lore: Warp test.
It all boils down to how much details and specialization you want in the game, I prefer simpler but each to his own. Just remember that having a skill only allows you to test it, not know everything about it.