How did as change semantically and ramify into all the meanings beneath? What underlying ideas or metaphors link them? Beneath, I chose only the broadest meanings from ODO, to see the "overall view". But for an even larger "overall view", I thank you even more if anyone can include and explain the numerous other meanings only on OED.
as = |adverb| 1. Used in comparisons to refer to the extent or degree of something
|conjunction| 1. Used to indicate that something happens during the time when something else is taking place:
Used to indicate by comparison the way that something happens or is done:
|preposition| 1. Used to refer to the function or character that someone or something has:
- During the time of being (the thing specified)
as (adv.) : c. 1200, worn-down form of Old English alswa "quite so" (see also) [...]
Etymonline is too curt. OED refers to linguistics and archaic Anglic languages, which I don't know.