Not sure what you mean by warm weather? They are just a shell. Can you sleep in your tent with no bag in warm weather? That is your answer.
The zipper generally, not on all bivies, but on most...zips you completely in. And if it broke while you were in and it was in zipped position you would have heckofa time getting out. Solution is not to zip it shut if that bothers you...
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http://ndeewoods.com/ and http://wilderstatepark.com/