Its the same old issue of lumpers vs splitters. At one stage Hunt in the UK put elegans and others as subspecies to M. haageana. In Germany, Reppenhagen kept them separate. Now Hunt has gone one step further by simply saying that M. haageana is very variable, and there are no distinct geographic separation of populations of the variously named "species". In Germany, Reppenhagen is followed more closely.
I have no idea who is right, but having been to Puebla and Oxaca, I have seen significant variation in locations where M. haageana forms exist. One plant might look like elegans, another like collina, another like conspicua......... I find it difficult to believe that such closely related species could exist as good species when mixed together so much. So, on this one, I tend to follow HUnt, but will stiull use the subspecies name, but as a form.
So M. elegans become for me M. haageana fma. 'elegans' almost as if it is a cultivar, which in a way it is.