No calipers? I thought everyone in the world had that exact same set of digital calipers...
So, you can see through the little circle that each filter screws into, but when filters are actually installed, the edges of the filter housing are cutting off the edges?
Which seems to be the case, as your most centered image showed cutoff in all 4 corners, correct?
To really prove that out, as you have no larger sensor dimensions available, I would probably take apart one of the filters and then just install the blank housing into the EFW. But that's just me lol, and probably not necessary.
I looked up the WO 81, and it seems to say it is compatible with FF and APS-C sensors. So, big image circle.
The question becomes, where are you cutting off the light cone, and what does the light cone look like?
I don't know a whole lot about light cones (except on the other end where I engineered a correcting mod on my 100ED), but I assume that somewhere it comes to a point and flips itself. You have tried a FR already and that is not straightening out the light path sufficiently.
So, can you stick the EFW in a position where the light cone is thinner? I mean, it's just a filter, and I presume flat across the entire diameter. In the four scenarios you previously posted, you had the EFW right up against the astrocam. If the light cone flipping point is somewhere to the front, maybe even inside the drawtube, then at your EFW location the cone is expanding once again. No? So, move the filter wheel forward? Is there any reason the EFW can't be right up against the WO81, or in various other places in the train?