If you are trying to just match the 2d movement of a layer, then you can use the match move behavior. It's basically a tracker but it works on animations built inside Motion as well.
as for the replacing media; yes you can.
There are a few ways. You can just drag and drop replace directly in the Layer List. Or you can right click over the clip in the canvas, sleect "Reveal Source Media" from the list and in the inspector on the media tab there is a button to "Replace Media"
Easy. Look at the Inspector of the object you want to copy and simply drag whatever attribute you want to copy to any other object in the Layers window. For example, drag "Position" of one object and drop it onto another object.