cloning a dev site into production

How can I clone a subsite of the parent used as a dev environment into the production parent environment and not lose all the production posts? They were coded with two dissimilar theme's one custom and the devsite is pure bootstrap based content. Lot's of hand made Sass css that would not be included in the production database which was made with divi so css is tattooed all over the database in production I am thinking.
I guess I should go look at the structure of the MU data as I am not familiar with it. Is the table prefix of the data the key to content here?
Will the cloning of the subsite into production rewrite the prefix of the dev tables to point to the prod data or import data that is called out with the dev prefix and therefore lose all content updated since it was cloned into production?
I am probably over complicated my thought process on this but I am sick of all night restorations from memory if I don't understand a process........