After I set my HDR and blur I cannot disable it and re-enable it without the screen turning into a small view in the top left corner. I'm not sure how I'm going to have the option to turn it on and off without requiring a restart of the game.

pp_set and pp_add are no engine commands, so it is hard to tell what might not work in your case. But you can check the functions in the mtl_view.c script and look for any issues with your other shaders or something else in your code. If they are not compatible to your project, use them as templates for writing your own pp functions.