8- Although it is not a child from the ImageTarget and shouldn't be augmented i find it related to the marker and change it's position based on marker position (This is my problem)
OK; so, you can update the Position and Orientation of your Object, you can find the ImageTargets TrackableBehaviour at runtime, then get the Transform component of it, and extract the position and orientation from that Transform, in World Coordinates (i.e. using Transform.position and Transform.rotation):
You can then set the position and orientation of your Object by deriving it from the one of the Trackable (plus adding any custom Offset translation / rotation).
If I understand correctly what you explained previously, even though there is no parent-child relationship between that specific object and the Trackable, however you are trying to set the position of your object by deriving them from the ones of the Trackable.
However, as soon as the Trackable gets out of view (tracking lost), the position and orientation of the Trackable might get to an invalid / obsolete state; as a consequence, if your Object position is somehow mathematically derived from those of the Trackable, they will also be affected by the loss of tracking.
In case you are using Extended Tracking, the Trackable position and orientation will continue being tracked, which theoretically should ensure some "continuity" in the update of your Object position as well; however, if the environment does not provide a sufficient amount of features for Extended Tracking to work properly, then your Object might also be affected by this, as long as its position and orientation are derived from /related to the ones of the Trackable.
In addition, there is also another element to keep in mind, i.e. the use of a specific World Center Mode; perhaps what you observe is also a consequence of the fact that, as soon as your target gets out of view, the World Center suddenly changes from the Target to the Camera; this might also explain why there is a sudden change of position and orientation.
To verify if this is the case, you can select the ARCamera in your Scene, and set the World Center Mode (in the Inspector) to CAMERA (instead of FIRST_TARGET)
Hope this helps.,