Hi there,
When I add CSS to the Additional CSS box in the Customizer (managed WordPress account) will that CSS stay in place if the Theme is later updated?
Yes
Does adding CSS in the ADDITIONAL box and in boxes provided by the SIMPLE CSS plugin create a sort of Child Theme that carriers over when the Theme upgrades?
It’s not a child theme but yes they will be saved through updates.
I would recommend use either Additional CSS or Simple CSS and not both so the code is all is one place and easier to manage.