>Why are angels proper nearly always depicted with wings in art, when no angel is mentioned in scripture as having wings?
Same reason why they are always should be male, to make metaphysical physical.
Angels were created in and live in Heaven, so giving the brids' (who fly in heavens, or sky, why do yall even have two words for that?) wings is fitting. They are also spirits, and spirit and wind is the same word in Hebrew and Greek, so again birds come to mind. They are not bound by gravity for they have no bodies ergo they can fly. And they are fast. Really, really fast. Speed of though fast.
>What experiences with angels are found in the lives of the saints?
Book of Tobit. Read it. There is also St Cecilia and her angel of purity, pope Gregory (?) who saw Michael at Rome or St. Aquinas who were given his purity cord by two angels after he kicked out whore from his room with burning rod.
>What led Augustine to conceive of the angelic hierarchy as he did? Was it revealed, or was it only speculation on his part?
Friend of Paul, first mystical doctor, St. Dionysus.