That idea is needlessly complicated... could be mine
If you just want your positives on Instax paper, it's probably much easier to put the slide into a projector and take a photo of the projection with the Instax camera.
Aside from needing absolute darkness, I think the film has a pretty high sensitivity, so you'd need a very short exposure, even with closed aperture. I don't know the exact numbers, but a ND-filter for your enlarger could be useful, if you still want to try it.