Hello again,
Regarding point 1, can you check if you did the following customizing :
Interaction Center WebClient->Agent Inbox->Map Item Attributes to Inbox Attributes ->Map Business Transactions to Responsible Employees and Groups
You should map the partner determination procedure used for you transaction type "call back appointment". The error you get can occur because of missing customizing.
Regarding point 2, when clicking on interact button, the identification of the business partner is supposed to be done automatically.
Best regards,
Sylvain