I haven't done any reading on this at all,but what puzzles me here is that I've always assumed that an offset is a clone of the host plant. genetically identical. You spoke of crossing two variegates,but in crossing them,you are intending to collect seed(which will have it's own unique genetics resulting from from sexual reproduction).

My guess would be that if a konjac pup was reverting to all green from a formerly variegated appearance,it would have to be a result of some cultural change like lighting for example,but I'm shooting in the dark here.

Hopefully someone who can shed more light on this will show up soon and offer their two cents. :)

It could be that SG displays chimera variegation, meaning the plant has one set of genetics that produces chlorophyll, and one set that doesn't. So an offset could come from part of the corm that only has the chlorophyll-producing set, resulting in an all-green plant. I've also heard of an all-white SG which, predictably, didn't do so well. I am not aware of an SG flowering, but it may have happened.

Nightstick is probably a very different sort of genetic mutation. I've grown it for a few seasons and the offsets show the black stem. Whether it's recessive or not, or even caused by multiple genes, I don't know, and I doubt anyone has done serious research into it. Someone crossed a nightstick with a normal konjac a couple of years ago, but the last I heard was that the seeds germinated successfully.