Okay, but keep in mind that I wasn't an english major, that english isn't my first language, and that all of this comes from nothing more than a knack for understanding languages and their templates. I only speak three languages fluently and I don't practice enough for the fourth. I don't know if any of those rules are hard-and-fast; they're just my observations. I retain the right to be wrong.
Firstly, "imparsable" is most likely to be correct (eg "impossible", as has been pointed out), however, something about that word rubs me the wrong way. Try saying it and you'll hopefully understand what I mean.
Secondly, "parsable" has to be correct, because the 'e' gets dropped (eg "judgmental"). I can't think of any counter-examples, but the fun part of language is that a counter-example does not break a rule... It's merely an exception.
As far as my idea, "unparsable". Now, we go back to what you said, and I believe that the reason you are right is that 'p' is a hard consonant (unlikable, etc). You can go to
and look up 'unlikable' -- it'll offer you to look at "un-" . The definition will, I think, prove you right.
Regardless, based solely on the fact that language has to be spoken, I can't regard "imparsable" as correct, while "unparsable" rolls easily off of my tongue.
That's all I've got, unless I think of something more later..
-Haben sie fosforos?
-No tiengo caballero, but I have un briquet.