I believe that you can tweak nanites setting to get some better result. The default reduction might be too low for your need.
About the environment, what I meant was exactly it. If you’re used to render in Arnold, then I’m happy to tell you that there is a plugin for UE called ‘‘HDRI Backdrop’’ which you can use for amazing renders!
Also, in your movie render queue when you render your scene, you enter in ‘‘play mode’’. So if you want to see exactly what the MVQ will render, hit the play button. Sometimes it can be a level that is loaded ‘as blueprint’ (So it will not load in play mode - that could be a reason for your post process not rendering)
You can try using the post process in the camera directly instead too (Same settings as the post process volume, but only attached to the camera)
After all this, if possible to see your render setting from the MVQ it give some useful information 
J’espère que ca aide un peu