Can a project partner declare the travel costs of its president under the category “External Expertise and Services (EES),” knowing that this person is neither an employee nor a staff member of the organisation? Or are these costs also covered by the flat rates?

The travel costs of the president (or any other member of the partner organization) cannot be declared as real costs under “External Expertise and Services,” even if the person is not an employee.
Any person belonging to the partner entity is covered by the flat rate, regardless of whether staff costs are declared or not.
As specified in the programme manual under the “Travel and Accommodation” category, costs for decision-makers, administrators, or managers participating in project activities fall under this category and are included in the flat rates.