I'm not sure the word is actually of "pure Adunaic descent". Though Akallabêth is said to literally mean 'the Downfallen', in the commentary for The Drowning of Anadûnê (in volume 9 of The History of Middle-earth) Christopher Tolkien suggests that the Akallabêth derived from a 'Mixed Dúnedanic Tradition, by which he seems to mean that it developed from a combination of Elvish and Númenórean accounts. The word does have similarities and possible roots in the Elven tongue.
For example, in Athrabeth Finrod ah Andreth - translated as "converse of Finrod and Andreth", we find beth, a soft mutation of peth, "word(s)" may have been the root to mean "story, record".
In Quenya & Sindarin, the verb "to fall" is lant, and cala means light.
Finally, a - translated "Oh!".
So we have a-cala-lant-peth as a basis to mean "oh!, the fall from light told". With soft mutation and natural elimination of identical syllables in sequence, this renders a-cal(l)ant-beth, which in Aduanic may have evolved into Akallabêth - and it's common meaning "the downfallen", from which kalab meaning "fall in ruin" may have derived.
I don't have any hard evidence, but I do find the similarities fascinating. But even if the root of the word is indeed Elvish, the stress would be on the first syllables as MLP suggests.