In terms of placement, the current setup with the new brig is working well I think. Interrogation actually gets used and the bigger processing room is used more as a holding cell for quick searches and clarifications/questions, since any items can easily be but in the nearby new evidence room.
In regards to the pileup, I think that's due mainly to the fact the for the more important cases, you often will have an officer or two, the Warden, and the HoS all in the big processing room with the perp. They don't all need to be there, but of course people are curious and want to see what happens. Then you get officers bringing in people for more minor crimes, and seeking clarification or at least acknowledgement from the Warden or HoS as to the charges and whatnot, which is where you see the pileup.
As usual, this can be helped by good record keeping. It really does streamline the whole thing if people dont have to keep asking about charges and crimes. Not even just the crimes, but the sentence, anything important to do with that case.
As an addendum: I've been thinking about working on some changes to the sec records system, or at least adding some more functionality to the sec PDA's in terms of records. For example a list of current warrants, that kind of thing.