Learn Russian: Кто-то включал компьютер в спальне? - Did someone turn on the computer in the bedroom?

This is the classic annulled result use of the imperfective past. The speaker is asking if someone turned the computer on, implying that it is currently off. If they used the perfective включил, it would mean the computer is still on right now.

Does using включал mean the person was interrupted and didn't finish turning it on?

No, the action was completely finished. The imperfective past is used here specifically because the result was reversed. The computer successfully booted up, but then it was shut down again before the speaker asked the question.

In a question like this, shouldn't it be кто-нибудь (anyone) rather than кто-то (someone)?

You will often see кто-нибудь in general questions, but using кто-то here changes the tone. The speaker isn't asking a purely hypothetical question; they likely see physical evidence that the computer was used (like a warm monitor) and want to know who specifically did it.

Why is в спальне at the end of the sentence instead of at the beginning?

In Russian, the most important new information usually goes at the end of the sentence. By putting в спальне at the end, the speaker might be specifically wondering if the bedroom computer was the one turned on, rather than the one in another room.