I doubt you will get any more knowledgeable people on another thread than those that frequent this thread. It may just take longer for people to see it and comment.
A possibility might be to contact your state ag dept or veterinary college and see if they will take a look at the photos via email and give you an opinion. The difficulty is not being able to see that structure in it's entirety and understand it's exact location in the body cavity and feel it's texture and confirm that it is not the kidneys, by locating those separately. Has the carcass been disposed of?
I was beginning to doubt my concerns about the amount of fat after reading Sue Gremlin's post, but I have done a bit of research since then (You Tube "processing laying hens") and now stand by my opinion as regards the level of fat, particularly that covering the intestinal membrane of a 10 month old pullet. I really do not think that is healthy.
Good idea Barbara! Sending photos to a Vet College or even an avian vet may be helpful.
I did see that dark mass in the photos, but was just not quite sure what to make
of it

I also agree with you on the amount of fat in the abdomen. I still feel like it's a lot, especially for such a young girl. The way it was attached to the intestines, I have not seen that either.
@Treerooted a few of the others are not online that frequently, so hopefully they will be by to take a look and give you their opinion. I wish we had better answers for you, thank you again for posting the photos. If you do happen to get someone to look at your photos, we would appreciate your sharing what you find out.