Yes, you can create as many headers as you want with different menus assigned to it and you can assign a header to specific pages.
Here is an intro video for the header and footer builder that you might want to check:
You can still get support here in Apex when you upgrade Pro. Even when your Envato support expires, automatic updates will be forever available as long as you have the site validated.
Demo contents are not available in Pro but you can make use of the Design Cloud feature.
Here is a link about the Design Cloud:
Your content should remain as is when you do an upgrade but it is always best to keep a full backup of your site before doing anything major changes.
Hope this helps.