The Chocomel shots look to me as if the f/8 has missed focus, plane of focus is an inch or so closer to the camera than intended (look a the blue wood, lower left of the Chocomel for the sharpest zone).
I've seen fixed-out or just sacrificed film sheets cut with corner slits to hold the film (on YouTube), works like album pockets but the film still goes into the slots in the holder and as such holds the Instax sheet against the septum. And you can locate the cuts to center the Instax where you want it, within limitations of the pod and upper frame vs. the film slot. Still, with the Instax sitting on top of the film sheet, you get enough extra thickness you might well have to recalibrate your focus. One of my 9x12 cm plate cameras has dual infinity stop settings, for glass plate and sheet film in a sheath -- the film sheaths are retained under the same spring clip that would hold the plate, so the film is further from the lens by the thickness of the sheath's metal. Solution was to set the infinity stop back that same thickness for film, relative to its position for glass plates. Seems as if one or the other would still focus wrong with the ground glass, though; it might be a setting that was intended for a rangefinder that my camera doesn't have (and never did, as far as I can tell).