I have my own thoughts on these. These marks actually say 'Khnum-Khuf' rather than your 'Khnum-Khufu' -- a small point but one that may have importance such are the vagries of AE script.
These marks could have been faked or they may well be genuine. Let me put it this way--I am more convinced that the Khufu cartouche in Campbell's was faked by Vyse than I am with these marks. But they have to be tested to settle the question.
As for Khnum-Khuf v Khufu--we know Khufu transcribes to Suphis. What does 'Khnum-Khuf' transcribe to? Has anyone ever read the full Khnum-Suphis name? Is it written anywhere or are we just assuming that Khnum-Khuf is the same person as Khufu?
I don't really have much more to say on this matter for the moment--at least not until these markings have been scientifically authenticated (or not).