Another piece of circumstantial evidence raised by Dr. Mortimer in his biography of (Roger) Mortimer relates to the title of Prince of Wales. Since the mid-14th century, it has been a title given to the eldest son of the reigning monarch, but that wasn't the case initially. It was a title created for Edward II (by his father) and he treated it as a personal title: Edward III was never Prince of Wales. Nor, for that matter, was Alphonso, Edward II's predecessor as heir apparent who would have been in line to take the title had it been doled out immediately following Llewelyn's (or Dafydd's) death (the future Edward II having not even born at that time). Edward II was granted it and held it for the rest of his life, even after he surrendered the title of King.
Looking at the title progression of Edward the Black Prince, he was created Earl of Chester in 1333 (aged three), which was informally the traditional title for the heir to the throne. He was then created Duke of Cornwall in 1337 – the first English dukedom in history. But he wasn't created Prince of Wales until 1343. The gap between Chester and Cornwall is easily explicable: Cornwall was a new title and the creation of English dukedoms an explicitly political act to establish the English crown on a par with that of France, which Edward III was simultaneously claiming. But Prince of Wales was a title already extant within the English peerage and moreover one with a degree of domestic political symbolism. And Edward only created one dukedom in his initial foray into the territory; he had no other sons yet, and delayed giving one to his cousin Henry for a further fourteen years. So why not give Edward the title of Prince of Wales? Why invent a new title instead, and wait so long to invest a new Prince?
It's possible that Edward considered that the title had a degree of political responsibility attached to it, and so preferred to wait until his son was a little older before granting it (contrary to the popular legend, Edward II was sixteen when created Prince of Wales). But in that case why not take the title himself upon his father's death, as he did with the title of king and other offices? Moreover the title under the English kings was always largely symbolic, and almost certainly had no more real political heft than did the duchy of Cornwall, which Edward III was happy to hand to his son at the age of seven.
Dr. Mortimer's explanation is that Edward II survived until 1343 and it was only at his death that the title became available once more. A death in 1343 would make Edward II almost sixty, roughly in line with other near-contemporary male Plantagenets who reached adulthood and died non-violently. If I remember rightly Dr. Mortimer also notes that some of the records suggest a "Welsh" origin for the pilgrim, and in Edward III's meetings with the mysterious hermit again referred to him as a "Welshman", which correlates with his remaining the incumbent Prince of Wales after his deposition and supposed death.
It's an interesting theory and like many conspiracy theories I kind of want to believe in it, but much of the evidence for it is thin and circumstantial, so I have to treat it with a healthy degree of scepticism.