"Who is in the office?"
Put 里 at the end. The meaning really who is at the office, not who is in the office.
There is absolutely a need for it here. Without 里 the sentence is grammatically incorrect.
Sorry, that's not really correct. I know the difference between 内 and 里 can be confusing, but once you get the hang of it the difference is easy.
It would make the sentence very formal, almost archaic, sounding. I would add 里面.
I've never heard that sort of construction (using 有). However, 办公室里的人是谁 would work.
Yes, that works perfectly, and I think the answer from @Jerry41034 works but with a altered syntax that means "Who is the one in the office?"
There is no need for a double character expression if a shorter one has the same meaning. Language economy is important