Sorry for the delayed response, and I’m sure it’s too late now to be of help now, but when one starts to zip, and makes good progress (at least 2-3 inches worth of “actual” zipping) then if they stop for more than 15 minutes or so at one time, I go ahead and break the cap off and let them finish pushing out on their own.
I have had ones stop zipping half way around the shell and die. My lavender orpingtons were really bad about it.
So in my opinion, once a full zip is in the works, they are ready to be out. They should only stop for small pauses, the zip should only take 1-1.5 hours at the most, from start to completion.