I think the purple edge could be due to stacked filters with a wide angle - it's probably shadowing onto the lens.

I've tried stacking 3 filters at 24mm...no!

If you look around the edge, you can see light drop off in all the corners. Must say that the grad doesn't look particularly strong, whilst the warm up does - I still think the warm up is actually too magenta. 

And after all that ranting - must say this is a bloomin good capture.

I used to get vignetting on my 90mm schneider...the image circle was a little shy to cover 5x4" with movements...

But it often made the photo!

It was.