How can the earth ground loop be modelled for this case?
Ok, you clearly showed some effort and so I give you my 50 cent. 

I suppose to consider only nodes ABCD as ground loop, because BCXY might typically be two wires in a cable connecting two devices. So this loop is usually small in area compared to the „classy” ground loop of the two earthed devices. (And yes, size matters in real world :) 

Also BCXY is not a ground loop by definition.

A ground loop is always susceptible to time variant magnetic fields, which induce a voltage V in the loop (principle of electrical motor or generator). So you should insert a 50Hz AC voltage source in your model with some amplitude V. The question is if this V source should be modeled between AB or CD? 

Please share your simulation results with us.