How to get all messages expanded in conversation mode in Outlook 2013?
Solution 1:
Just check Always Expand Selected Conversation in Conversation Settings menu
(right below Show as Conversations check).
Solution 2:
After checking Always Expand Selected Conversation in the Conversation Settings menu, click the Change View menu item and choose Apply Current View to Other Mail Folders... to apply the Conversation Settings to all folders.
Outlook - Change View