Problem:

Can I change the custom Field Name and Field Slug?

Solution:

Yes, you just need to change the custom field settings: Field Name and Field Slug, you don't need to re-save your posts.

Relevant Documentation:

Problem:

The problem here is that the Toolset custom fields were not showing up for Rank Math to add the fields to the page's schema.

Solution:

The Toolset Custom fields are stored using a wpcf- prefix in the post meta table so if when searching for the fields you will be required to search with the wpcf- prefix.