I use a different means of getting the x, y coordinates. I query the layout Dom to get hat info. Here is a typical query:
var obj = formname.pageName.objectName;
var oY = xfa.layout.y(obj, "in");
If you need to set the x,y value of an object you do not do it through the layout Dom. You would use:
objectname.x = "10in";
I see you trying this with a signature .....note that a signature is not really an object in XFA but a placeholder that is transformed into a signature when a PDF is created. I would wrap the signature in a subform and operate on the subform instead.
Hope this helps