@Earthman Web & Media, I am deliberately rude to WordPress insiders, because their responses/solutions are usually useless. I hope to get responses from regular, real-world WP users struggling with the same frustrations. If not, my posts help me keep track of unresolved issues.
I think your solutions also miss the point again - correct me if I'm wrong. In my custom theme Firstname Lastname are already consistently stored, but new blogs and user accounts still sometimes have display name format set to 'crapusername'.
I am looking for a catch-all solution to make Firstname Lastname the default display name format, across a network, forever and ever. Storing Firstname Lastname from a form is not the problem.
Gravity Forms and various user registration plugins are not solutions to any problem having to do with registration - I even made this point in my original post. As far as I know Gravity Forms is mainly a way to make adding forms to WordPress "easier" for people who don't know any PHP.
I am especially rude about Firstname Lastname and the entire registration process because WordPress clearly has no intention to fix it, bring it up to current internet standards. I have been asking about these things for years, since at least '96, initially friendly, by now not so much.
One response I got from an Automattic developer was that WordPress couldn't do Firstname Lastname, because it was insensitive to certain unnamed cultures that don't use that name format. WordPress lives in a weird bubble for nerd bloggers that never go anywhere.