Before I even submitted the captures, we saw that all the spacings produced (apparently) the same FOV.
There was no noticeable change. The capture app did not change the field dimensions. All were at the max that you stated.
When I submitted the images, I first converted from tiff format to jpg.
Then (to speed up the solving time) I resized the images by about half.
The FR/FF is doing a great job of flattening the field.
I have perfectly round stars to all the corners. There is no coma, so at least that is great.
So your spacing was less than half of what is suggested. That would explain the rather small reduction.
The flattening part may not be a big issue yet. The 1600 measures only about 22.2mm diagonal, so you're still mostly using the center of the field produced by the scope, where it should be fairly flat anyway. The images you take without the reducer will show some curvature, and if the reducer didn't correct for it the reduced image would show a lot. So the real test will be when you get the spacing closer to 55mm.
Making progress. Keep at it.