Bringing this back up with a new theory regarding a full translation for Nahfahlaar.
There are two main possibilities: nah + fah + laar and nah + fahl + aar, that is, "fury for ___" or "fury ___ servant."
The first is convenient because it would mean laar is shared across all three of the above names. The second, though, makes sense given Nahfahlaar's history of being a servant of Tiber Septim, and allying with other mortals.
Fahl at first glance seems like an impossible word since ah is featured at the end of syllables, not in the middle. By the rules, it should be spelled as faal, which is already a word. Well, there are two canon examples that contradict this rule: wahl "to raise" and nahl "living." Fahl fits right in. The fact that it's a homophone with faal doesn't outrule the possibility, since nahl is a homophone with naal "by."
So, fahl is a possible word. But what does it mean? Turning back to wahl and nahl, both originate from longer words: wahlaan "creation," also the past tense "raised/built," and nahlaas "alive." They are pretty direct relatives.
The only known word that could match fahl is fahliil "elf." To go further, we'll have to dig into some grammar.
There are possessive suffixes that can stand for pronouns like "my," "our," and "your." One of these is -u, and is used to form Bormahu. It refers to Akatosh, and means "our father." The one that interests us is -iil, which means "your."
The crux of this theory is that fahliil could be a construction similar to bormahu; "your ____." Looking at similar words, the one that strikes me is paal "foe." What if the word for "elf" was originally a construction that meant "your foe," referring to the elves as the foe of the Nords? In this scenario, fahliil is a corruption of an original construction paaliil. Fahl, then is an obsolete equivalent to paal.
This makes Nahfahlaar "fury foe servant."