Use robot_localization to integrate fixed overhead camera with wheel odometry

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Hello, I'm trying to integrate the wheel odometry's measurements of a Sphero robot with the position measurements obtained from a visual tracker algorithm (TLD) running on the stream of images of an overhead camera (Kinect v2).

Since the visual tracker algorithm is giving me the 2D position of the robot, I was thinking to use the 'PoseStamped' input message type.

The sphero's odom message contains position and twist (orientation is always the identity quaternion); I'm using the sphero ros package.

I'm trying to use the robot localization package, but I'm a little confused and I have some questions:

1. What does 'absolute' measurement mean? It is relative to the 'world frame'?
2. The visual tracker sometimes tracks false positives in other points of the image, and sometimes loses completely the tracking, so it's right to say is subject to discrete jumps. The position obtained from the visual tracker is in a frame called 'world frame', and the position obtained from the odometry it's in another transform tree, in the odometry frame. I need the transform from the world frame to the odom frame, so, it is right to set the map frame and the world frame to the same value (world frame)? Do I need to start more than one instance of the ekf localization node?
3. In the end, what's the best strategy to use in my case for the sensor fusion?

This is my launch file:

<launch>

<node pkg="robot_localization" type="ekf_localization_node" name="ekf_localization" clear_params="true">

  <param name="frequency" value="30"/>

  <param name="sensor_timeout" value="0.1"/>

  <param name="two_d_mode" value="true"/>

  <param name="map_frame" value="world"/>
  <param name="odom_frame" value="odom"/>
  <param name="base_link_frame" value="base_link"/>
  <param name="world_frame" value="world"/>

  <param name="transform_time_offset" value="0.0"/>

  <param name="odom0" value="odom"/>
  <param name="pose0" value="robot_2d_geometry_pose"/>

  <rosparam param="odom0_config">[false, false, false,
                                  false, false, false,
                                  true, true, false,
                                  false, false, true,
                                  false, false, false]</rosparam>

  <rosparam param="pose0_config">[true,  true,  false,
                                  false, false, false,
                                  false, false, false,
                                  false, false, false,
                                  false, false, false]</rosparam>

  <param name="odom0_differential" value="false"/>
  <param name="pose0_differential" value="false"/>

  <param name="odom0_relative" value="false"/>
  <param name="pose0_relative" value="false"/>

  <param name="print_diagnostics" value="true"/>

  <param name="odom0_queue_size" value="4"/>
  <param name="pose0_queue_size" value="5"/> <!-- default 10 -->

  <param name="odom0_pose_rejection_threshold" value="5"/>
  <param name="odom0_twist_rejection_threshold" value="1"/>
  <param name="pose0_rejection_threshold" value="2"/>
  <param name="twist0_rejection_threshold" value="1.2"/>
  <param name="imu0_pose_rejection_threshold" value="0.3"/>
  <param name="imu0_twist_rejection_threshold" value="0.1"/>
  <param name="imu0_linear_acceleration_rejection_threshold" value="0.1"/>

  <param name="debug"           value="true"/>

  <param name="debug_out_file"  value="debug_ekf_localization.txt"/>

  <rosparam param="process_noise_covariance">[0.5,  0,    0,    0,    0,    0,    0,     0,     0,    0,    0,    0,    0,    0,    0,
                                              0,    0.5,  0,    0,    0,    0,    0,     0,     0,    0,    0,    0,    0,    0,    0,
                                              0,    0,    0.06, 0,    0,    0,    0,     0,     0,    0,    0,    0,    0,    0,    0,
                                              0,    0,    0,    0.03, 0,    0,    0,     0,     0,    0,    0,    0,    0,    0,    0,
                                              0,    0 ...