I'll weigh in with my 2d-worth.
It can be difficult with a young plant,as you clearly understand. M. mystax is supposed to have between 1 and 7 central spines as well as 3-10 radial spines. M. karwinskiana rarely has any central spines, and the radial spines can be quite variable in number (3-10) and length.
Reppenhagen described a plant he found in Oaxaca as M. karwinskiana v. longispina, and I have a couple of young plants, but the spines aren't as long as those of your plant.
Based on the spination, I'd support karwinskiana, but the spination does look pretty extreme for that species, as Delandmo has already said.
It might be that central spines will develop as the plant ages a bit, that's not uncommon, and certainly the flowers when those appear will help a great deal.