The Arnold render settings is a Video Post plugin. To set up the render settings programmatically you have to create a new video post instance and add it to the active render data.
The CINEMA 4D python SDK has a limitation and does not allow creating video post plugins. As a workaround C4DtoA offers a command to create the Arnold video post. The command id is 1039333.
Here's a simple example code which returns the active Arnold render settings video post or creates and activates a new one if does not exist yet:
If you have the video post data you can change any render settings as you do normally by an object or material: