I followed your suggestion but it did not address the issue for Konqueror was already in the list of apps. Then I discovered this solution:
From Konqueror or Dolphin right-click on a directory and select Properties
From the dialogue find Open With
Click ‘Change…’
One should see a dialogue for File type inode/directory
Dolphin and Konqueror are each present in Application Preferences but sub-directories chosen in Konqueror open in Dolphin
Click tab marked Embedding
In that tab I found the option ‘Show in separate viewer’ set. How it became set that way, I do not know.
Choose the first option: ‘Show file in embedded viewer’