Orange peel

Here's a few things to consider. Wet sanding a single stage paint with mettalic in it will disturb the mettalics and it will show big time. You are sanding the mettalics and changing how they are sitting in the paint. Any time you sand a mettalic paint weather its single stage or a base coat it needs to be recoated to distribute the flake properly.

As far as the orange peel, if the temperature dropped it would actually assist in helping the paint flow out. It will slow the cure, and allow the paint to lay down after being sprayed.

Consider getting yourself a cheap HVLP Gravity feed spray gun to do this. Your results will be MUCH better. You can get a decent gun for $50-100. I'd suggest a 1.4 tip for your single stage.

Putting clear over something will not assist in this. The peaks and valleys of "orange peel" tend to pool mettalics in them, and just clearing over it you will still see this visual effect.

My suggestion, buy a cheap gun, block the panel down with 600 and respray it.