As Osburn was at Giza immediately after Lepsius, the engraved tablet at the entrance would have been freshly incised and should have been detected as such. Osburn, in "The Monumental History of Egypt" Chapter II, reveals a thorough, scholarly understanding of hieroglyphics. In Chapters III and IV, he mentions Lepsius, and reveals that he was familiar with his work.
As stated, he mentions the cartouche of Noh-Suphis being at the entrance- the Khnum Khufu cartouche - but nowhere on the Lepsius Tablet does that cartouche appear.
It's reasonably certain that Osburn had a far greater understanding of hieroglyphics than Vyse - he translates the Rosetta Stone in Chapter II. Therefore, I don't consider that Osburn somehow made a mistake in his observation.
So where did Osburn see it?
Could you please elaborate on your final remark,
"The ”earlier explorers of the pyramid”, including Vyse, were incapable of detecting a cartouche incised in 1842."
Lepsius was obviously familiar with the cartouche as he had six different ones inscribed on the entrance gable, including the simple Khufu cartouche in vertical format.