I think after the prashant death, ARYA went in search of Ansh leaving behind - Dubey, Shoba, SP and Pankaj AND ofcourse police. Now I think SP noticing the weakness of Dubey ( and the key in prashant pocket) might have been roped in to play the unfinished plan he chartered out for prashant - in the absence of Shoba and Pankaj.
As you said Dubey is not the kidnapper BUT one among many hand involved. Hope it comes out the way we all have chartered out because somehow it doesn't look convincing (ofcourse - we will be clear after todays episode) climax and I remember Yash words - ' probably we can trap the man (unwrap the gift) who will ultimately lead to the main kidnapper' Right?