Part/most of the two hours would be for Immigration and Customs, so whatever residual is left, wouldn't be enough to see much of Dandong even though it's not a big city. It makes no sense to me that once you've gone through Chinese entry formalities, that you'd be locked away from the dining car. Seems to me that the correct authority to ask would be the tour company taking you to and out of the DPRK.

However, one obvious solution is not to book the train any further than Dandong. Just arrange with the tour company to return as far as Dandong. Then you could get out after you pass Immy/Customs, look around all you want even stay the night. And make a separate ticket/reservation yourself, for a 100% Chinese domestic train or flight Dandong-Beijing. There's no need for you to stick with the tour group once you're out of North Korea.