(experience speaking here)
If you didn't manage to get the "co-host" pinned down to what she figured her part in it is, and considering the fact that she sent the invitations out late ... i am guessing that "co-hosting" was also HER idea!
Do what you want to do, in any manner you desire to do it .... and, keep her name in the frontal lobe of your brain as "doesn't play well with others" and don't bother to try again ... she has already told you what she thinks about "work" or "ethic" ...
Having just re-read what you asked: I don't think you are being harsh, and you can drop the word "co-host" from your introduction of yourself. And, I am with you: If you have to ask her to do what "polite" people know how to do, then she isn't really helping, is she?
However, do not allow yourself to be bitter ... just learn from the experience and rest assured that there really are people who will work well with you. Practice shrugging your shoulders, and let this roll off your shoulders. Life is too short to try to fix everyone, though that is usually what my feminine person wants to do that ... one casual acquaintance down, and another friend on the horizon .... keep looking up! and, SMILE! People will wonder what you are up to! LOL