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Does the Edge HD still exhibit some coma at the edges (As in is it expected to?)
If it does, you could probably collimate it the same way I used to do my Vanilla C925. Focus on a centered star and then collimate to have all the "Comet tails" pointed directly away from the center of the frame. So edge of field stars in the middle of whichever edge of the image they are located should have the coma tails pointed directly left, right, up or down, depending on where they are in the image, corner stars with the coma pointed dirtectly in to the corners etc.
I found it easiest to align the camera so that the long side matched the "Flat" made by two of the collimation screws, to make it easier to work out which screws to adjust. I think I would do as Simon suggested and collimate first with the reducer removed.
One thing that is interesting to me is that the coma tails are opposite to what I always saw with my C925, with the shadow of the secondary to the outside of the defocused star, not the inside, perhaps an effect of the Edge HD reducer?
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