Don Mankelow - What is the protocol for preferred names?

Middle name is for the middle name.
Names that a person also went by go in the Also Known As field.

If you want to strictly control how the name is displayed, use the Display name field. Individual users can set their settings to ignore the Display name field.

Geni, or was it My Heritage? was suggesting because the Wiki Tree profile has his name as 'Donald "Don" Mankelow'
that this indicates this profile has more information, as in a 'middle name'
(Don was the last of 12 children and all his siblings have middle names)
So this might be Geni software code can't handle such variables

It was MyHeritage making that suggestion. Keep in mine the quality of the source data at MyHeritage is based on what individual people do with their private trees. That data may have come from a tree in which they decided to use the middle name field for people nicknames.

thanks for your thoughts. I guess I need to keep in mind that other genealogy programs like Wiki Tree, might well have a 'nick name' field but chooses to display that alternate name in brackets - much like maiden names.