Where Esperanto is weird here is that the adjective must then become an adverb (Lavi estas bone), following, I believe, the usage in Polish, but ignoring the usage of most languages in the world
No, in Polish it's also an adjective. It most likely follows Russian, which has coinciding forms of neuter short adjectives (generally used as predicates) and adverbs, and Russian grammarians decided that in sentences of that type predicates should be classified as adverbs, for some reason.
Also, it is a very weird feeling to call such a positive thing as "open" - malfermi - by two "negative words". But maybe that's just my German way of thinking...
Why, German also has "aufschließen". And Russian basic word for "open" follows this pattern too: открывать = out + to hide (similar to Italian scoprire and English discover).
Not sure of the etymology of "jen"
You may find useful this Etymological dictionary of Esperanto
jen = Lat. en
I'd like to mention one more resource for learning Esperanto (I know they are a plenty, but this one I particularly like): http://www.youtube.com/watch?v=mWbyXVSiCxw
It's the hilarous cartoon Muzzy in Gondoland originally made for learners of English. I particulary like the pronunciation in it, it sounds very natural and nicely Romance-like.