The structure you describe is very good and as far as the BS Liabilities Trust Account you could use the Customer Credits accounts. Create your Clients under the Customers tab.
When you receive money from the client enter their name as Payer and the Account as Customer Credits
When you pay the tour operator the Account would be Customer Credits - Customer
Then when you click on the Customers balance you can see the details
For clarity you could rename Customer Credits to Trust Account
For the Commission just create a Sales Invoice - which would clear out any Client Balance