I came across a reference to the accentuation of -αι and -οι at the ends of words that said something like “while accented short, as diphthongs, these syllables were pronounced long.”
Smyth states the following: "The difference in the quantitative treatment of -αι and -οι depends on an original difference of accentuation that may have vanished in Greek. -αι and -οι, when short, were pronounced with a clipped, or simple, tone; when long, with a drawled or compound, tone.
So I’ve always tried to give final -αι and -οι a short value in Attic prose, but a long value in reading Homer. What is the metrical evidence for their length in Attic verse?
(Strangely accented -εως words like πόλεως are a product of transfer of quality from ηο → εω, so a completely different phenomenon.)