Your post serves as a good reminder of why common names are frowned upon the more you learn in this hobby.
N. pulcher has a double-slash, "\ \" style marking on the gill plate.
N. brichardi has a 'T' style marking on the gill plate.
N. helianthus are typically a bit darker than the above two species and have a 'V' shaped marking on the gill plate.
Of course, if you're buying small fish they might be hard to tell apart and trusting the source of your fish is important.
Personally, I've always known "Daffodil" to be N. pulcher and would reject any other premise without a really good argument.
With regards to the names you found, the ones containing "Lamprologus" are the older names. In the last 20 years or so what's known about cichlids from a taxonomic perspective has changed a lot and a number of things were reclassified. Neolamprologus is the newer genus naming for these fish (and many others previously known as "Lamprologus".
Adding a twist to all of this, sometime in the last 5-8 years there's been new evidence that N. pulcher and M. brichardi are the same species technically, just different locations that resulted in a coloration difference, but not enough to warrant a new species. I'm not sure if they've been formally reclassified yet to a single name (or if they will be).
