There's huge change coming to Roshar, and I doubt that it's going to be feasible to try and restrict these "mythical" Radiants by law. Of course, some will try, but I don't think the Vorin countries will be much part of that. (Except for maybe Jah Keved. Who knows what Taravangian is gonna do?) Thus, I don't think a pardon is needed. We'll find out in Oathbringer just what Dalinar has done, and Kaladin has done some stuff too. It's not going to be realistic to convict all these people who are prophesied to save the world (or something).
As to Nan... He seems to be limiting himself to obscure proto-Radiants at this point, like Ym and Lift. Shallan's a little more well-known and would probably be harder to kill, especially now that she's realized her powers and actively uses them, while knowing what she does is Surgebinding (at least mostly). Eventually, things may happen and you're right, Nan may eventually see what Shallan has done as grounds for another murder, but right now she's safe. I think.
That probably won't happen for a while, but it's a really intuitive thought. Kudos to you!