Q 2. Which documents should be provided in order to properly reimburse an official travel expense?

In order to properly precede to reimbursement of official travel expenses, a proof that the person did the trip must be provided.
In the first place, boarding passes should be provided. However, as nowadays tickets are most of the time electronic, therefore, apart from the electronic ticket, the person should provide further tickets or invoices that proof his/her presence in a country/city such as restaurant, taxi, local transport... tickets.
In short, boarding passes should be presented if possible. If not, the electronic ticket must be provided together with additional tickets (electronic tickets only do not proof that the person really travelled).