The one glaring omission from DH is the Inquisitor in training. The Explicator/Investigator is kind of a core part of both the BI novels AND the TT fluff - yet I don't see a career/background/options for this career.
Indeed, but you have to remember that Dark Heresy is a game focussed on playing the mooks, redshirts, and lowly expendable indentured servants of the Inquisition. Only such 'canaries' that survive their trials long enough to 'attain the final rank in their respective careers' (see Completing all ranks - DH p43) are considered worthy of even being offered the role of trainee Interrogator, and is its made clear on p43, that is beyond the scope of Dark Heresy...such characters are 'too powerful' for the DH rules to cope with.
The idea that Dark Heresy is a game where you play Inquisitors, or even trainee Inquisitors is, according to the rules themselves, incorrect.
That's not to say you can't sort thigs out yourself and make the PCs Inquisitors or interrogators, but the rules authors consider that to be beyond the scope of the rules they wrote.
I know that many have said that any Accolyte could be an Investigator later on - but thats not really how the fluff presents Inquisitor's in training who are supposedly picked from the moment that they would be entering the Black Ships if they are psykers or would be chosen from the Schola Progenum's greatest students.
Indeed. Wierd eh?
As a career option I think the Investigator/Explicator would be great - he could be to the Psyker what the Arbite is to the Guardsman - a social and investigation based Psyker with less raw power and more skills and talents. He might also have some unique talents based on his training from the Inquisition and his social benefits (ie Peer Group - Inquisitors)
Thats certainly a way to try and shoehorn such characters in, and if it works for you, go for it. I think you've got a fair bit of work ahead though, since Dark Heresy provides no answers to the following (all vital to sort out to allow PC Inquisitors / Inquisitors in training):
What authority does an Inquisitor have?
What responsibilities does an Inquisitor have?
How does he exercise these?
How does an Inquisitor marshal resources?