You've done well and it is nicely resolved!
I think the issue in the corners is to do with the corrected circle of the 0.63x reducer. You camera sensor may be larger than what the reducer is designed to correct for. Just means that you may need to crop out the edges.
It could also be that the spacing is a little off as you suggest. Checking the specs on the reduced and/or trial and error on the spacing will give you an answer
