WNW: Why Is "Colonel" KER-nel?

After my post about lieutenant and why the English pronounce it with an F like luff-tenant (the post, by the way, that sparked Word Nerd Wednesday), several people asked a similar question about colonel:
Why is colonel pronounced with an R and TWO syllables instead of an L and three syllables?
At the time, I had no idea. I've since done some digging (thank you, OED and Professor Google, but mostly OED), and now I have an answer for people as nerdy as I am (ie anyone who loves weird language...