Thank you, Si. My plant receives plenty of sun. In fact, so much that it has scorched a few plants this year. The sun is getting noticeably stronger, here in Portugal, and I find I have to move the plants around a lot in order to keep them growing or simply alive...
I'm glad it has flowered at last, never mind the fruits. It will get there, eventually. By the way, "dehinscence" is the "technical" word for the way a fruit splits open and is one of the traits often (always?) included in the description of a plant species. Apparently, Mammillaria fruits usually open by a basal opening whereas this species has a different habit.