As far as I can remember, ἕως + opt. stands for ἕως ἂν + subj. to express past repetitive/general action. ἕως ἂν + opt suggests the ἂν belongs to the opt. as standing for the potential optative. That's why the author had put the English equivalent in parenthesis, (until you might come), but the practice should have been the same for ἂν to avoid confusion.