How could we make rviz subscribe to a topic of arm_navigation_msgs::DisplayTrajectory msgs?

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You need to use the "Planning" display ("Add" -> "New Display" -> "Motion Planning" -> "Planning"). The description says it takes motion_planning_msgs::KinematicPath, but that's not true any more; it really takes arm_navigation_msgs::DisplayTrajectory messages´. I filed a ticket on that.

(Update: the "Planning" display is loaded via the motion planning RViz plugin, so you need to resolve your other problem first.)