Tinwolf, the reverse-ordering of phonemes that occurs is called
metathesis. The nicest example I know is in the Irish language (Gaelic): leprechaun was originally luchorpán (meaning small-bodied), or at least that's what my brother learned studying Old Irish. Try saying luchorpán over and over, and it naturally ends up as leipreachán.
(Just to show what a linguistic nerd I am, I always say me
thatesis instead of metathesis as an etymological joke!)
And yes, "I guess" is correct English, going back as far as Chaucer's Canterbury Tales (circa 1380s). Although, back then it was formulated like this "Four of the clock it was, so as I guesse". But I understand that the American usage of "I guess" (meaning "I think", "I believe", "I suppose", "I reckon") has a different etymology (according to Fowler & Fowler anyway).