Gershon,

In warm weather you will need to leave it unzipped or sleep on top unless the problem is bugs. If you close up tight in warm weather it will not typically breath well and you can get a bit of condensation. Then you will need to turn it inside out and air out both bivy and sleeping bag. During summer I used only a fleece liner and that dried quickly.

Yes the bivy does add a little warmth. Can't quantify, but with the Integral Designs version it seemed to be about five degrees. The very light one I use now, I think it's maybe two or three degrees. A lot of it is that it breaks the wind.

Never had a zipper jam in the closed position. If it did, zippers pull apart fairly easily, I think. Just push hard on it. You'd likely still be caught at the top, but you could wiggle out.

Hope that helps. best, jcp