You stated :
"The only thing I can think of resembling an arch (and of course it’s not a true arch) is the gable arrangement over the original entrance. This has the “fake” inscription by Lepsius on it."
Correct me if I am wrong, but didn't Vyse enter the pyramid through Ma'mun's forced entry. If so, no arches, even of the gabled variety in the downward incline were in evidence.