My textbook says that “τῷ ᾀεὶ ἐπιθυμοῦντι” means “person who is always interested”, but doesn’t explain why. Maybe that is because the grammar is untouched, but I wonder.
The difference between subscript and adscript is merely orthographical. ᾀεὶ is an unorthodox spelling of ἀεὶ, though it may serve as a reminder that the first syllable is always long.
And ἀεί doesn’t mean “always” so much as “on each occasion,” referring to a succession of different individuals, cf. e.g. ὁ ἀεὶ βασιλεύων.
I think Joel’s referring to the quantity of the first syllable of ἀεί. It’s true that it’s sometimes shortened by internal correption (when a long vowel gets shortened before another vowel). This happens even in Homer, and in Menander ἀεί at line end would necessarily shorten the alpha. At the beginning of Sophocles’ Ajax, on the other hand, ἀεί may be presumed to have kept the alpha long.
I looked through Euripides, and the first syllable of ἀεί seems always to be short or to more rarely go in the anceps position. Inscriptions after 361 BC always spell it ἀεί not αἰεί, per the LSJ. All this points to the first syllable mostly being short in Attic.
Presuming that it’s long in the first line of Ajax is based on nothing. That’s an anceps position. In Ajax 599 though:
At Aj.599 it’s not the antistrophe that determines the length so much as the meter itself. And its being spelled αιει counts for nothing in this lyric passage; besides, most of the manuscripts spell it αει.
This discussion has strayed too deep into the weeds to be profitable. We may disagree on the original pronunciation of αει as first word of the Ajax but I trust we’re agreed that in Plato the word would best be spelled αει.