Yes chickens and cows are very different, but well done for having a look. Sometimes taking photos of anything that looks odd and posting them here can help us to help you figure it out if a professional necropsy is not an option.
The crop normally doesn't smell bad, so there was definitely an issue going on there. Was the mucus in her oesophagus or trachea? Wondering if she had sour crop and tried to vomit or refluxed and aspirated, which then caused a respiratory problem.
Do you supply them with grit. If they are fed or have access to grains and seeds and/or grass and vegetable matter, they need grit to help break it down.
I've had a couple gorge on grass and end up with an impacted gizzard and literally starved to death because food couldn't pass through their system to be digested. I find it really interesting to investigate these things and it takes my mind off the sadness of the loss and gains me knowledge that will hopefully help me prevent similar losses.
Wishing you well with the rest of your flock.
Barbara