I have just seen your post re the Harley's and I am also a descendant of Aquila Harley through his daughter Grace who married Roger Stone. I was quite excited too, to see that we may have been descended from Robert, Earl of Oxford, but I have come to the same conclusion as you. Robert did not have a son Robert, and his son Edward, became the 2nd Earl of Oxford.
Have you been able to find the parentage of Robert Harley 1680-1747 or the confirmation of his death in 1747? I hope to hear from you and hope you may be able to shed some light on all this.