The thing is that in those consolidations quite some info gets lost. The books I have all suggest that M. karwinskiana ssp. karwinskiana can have 0-1 central spines and 5-7 radials ie. it can have a central spine and have 5 radiatls, which would make Daisy's plant M. karwinskiana ssp. karwinskiana given that this is a well known species that has been in cultivation quite long with a range that overlaps that of ssp. nejapensis also a relatively well known 'older' to cultivation plant one of those two as the main candidate is more likely than the two other ssp. which are in one case much newer and and in the older quite a bit rarer, with a distinctly different appearance. The books also suggest that ssp. nejapensis does not have a central spine, but how many specimens from what part of the range will they have looked at?
Anyway, Daisy I suspect your plant may be straight up M. karwinskiana ssp. karwinskiana iso ssp. nejapensis, as it would not be inconceivable for seeds from one to be conflated with the other. In addition labeling it nejapensis might have made the plant more desirable at one point given how karwinskiana is a more common plant with a wider distribution range. On top of that both plants are described as relatively variable so confusion between them may be unsurprising.
As to flower color... I have observed distinct changes in flower appearance from the same plant especially color wise depending on the time of year and the level of heat and humidity, so while helpful in distinction it is not definitive.
