If the site was created using ACF and the templates created by writing code to show those fields then you will not be able to use elemetor to alter those templates without completely rebuilding the site.
It is possible that the developer used CPT UI to create post types and then moved the CPT creation to code to improve performance. The same goes for the ACF field groups. However, I cannot say this is the case by reading your description.
The ACF Pro license only deals with updates. The version you are using will be completely functional without a valid license entered.
It could be that the developer hid the ACF admin so that the client could not mess with things https://www.advancedcustomfields.com/resources/how-to-hide-acf-menu-from-clients/
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