Dark Heresy isn't a combat game, so stop throwing gun bunnies and hordes 'o death at your PCs.
Try running a few sessions without ANY combat in them at all. Then the PCs probably won't die.
Of course a lack of combat doesn't mean there's a lack of conflict.
Take a bit of time to concentrate on the things the acolytes should be doing; investigation, surveillance, information gathering, networking, interrogation, etc.
I agree with your suggestion with regards to running the occasional session without combat ... but surely you meant this as a suggestion as opposed to the absolutes you presented... Dark Heresy can involve limitless scenarios, combat being one of them... and what the Acolytes should be doing is pretty much up to them... only the Sith deal in absolutes, and that's an entirely different gaming system