In principle, is it possible that those two spells are actually one and the same? For Fiendfyre, we have a name and are missing an incantation; for Protego Diabolica, it's exactly the other way 'round, if I'm not mistaken. It'd be an elegant way to account for the obvious similarities, anyway. Are there any precedents for name and incantation diverging that much?
Or maybe there's no such thing as "THE fiendfyre spell" in the first place? Hermione refers to it as a "substance" (quoted in the article) - which I always found weird and fitting at the same time, as fire is considered process rather than substance in modern terms, but not so in alchemical terms - which may mean that a whole variety of spells employ it in various ways. The spell Crabbe uses in "Hallows" could then simply be to fiendfyre what "Aguamenti" is to water, while "Protego Diabolica" summons the same stuff, but puts it to a more complex and controlled use.
To take this to the extreme, one could suppose that fiendfyre is simply the generic name for magically summoned fire which has the potential to take on "fiendish" shapes or traits or whatever, in which case it'd have been deployed a bunch of times throughout the books. That stance runs counter to the dire warnings attached to the term, though, I reckon...
Hermione erased herself from her parents memories so that they never knew they had any children. What if they had another daughter after that and named her Hermione because they didn't know that they already had a daughter named Hermione. How awkward that would be when Hermione came back and restored their memories.