To be fair to Kingfisher, when he listed Perceval and Parzifal as "eyewitness accounts," I don't believe that he meant that they were eyewitness accounts of the Grail. Rather, he was saying they are evidence that medieval authors recognized and used the so-called Genesis Seal. Of course, as others have pointed out, this claim is, at best, unfounded.
Like RobDegraves, I am a medievalist, though my area is English literature, so continental romances are not my strong point. However, I can say that Kingfisher's ideas about how textual analysis, literary interpretation and source studies work are faulty--to be generous. You don't take a random passage of one work (cutting it off in the middle of a sentence), rearrange it into some randomly chosen pattern and then look for stuff, yelling, "voila!" when you find the word "sword."
A smattering of shared words (especially when they've been rearranged) in different languages is not enough to say that the one work is a source for the other. It's not even enough to say that they are analogues that may be related in some way. Your word games do not amount to verbal echoes or verbal correspondences.
As for the sources mentioned by Chretien and Wolfram: it's a common literary motif in medieval works for the author or narrator to refer to some mysterious source. Geoffrey of Monmouth, for instance, claimed that his source for his Historia Regum Britanniae was a very ancient book in the British language given to him by Walter the Archdeacon. Geoffrey certainly had his sources, but very few scholars have much faith in the very ancient book. I believe Wolfram's source is also questioned. Certainly, there is no evidence for its existence. His main source is Chretien and his own poetic imagination. With Chretien, it's harder to say what the direct source is, partly because Perceval is such an unholy mess. In general, though, he is thought to have drawn on Celtic sources.
This sort of "literary analysis" reminds me of people who have discovered some sort of secret message or code that "proves" Shakespeare didn't write Shakespeare.