Your questions are great. In a case like this, I find it easier to deal with a concrete situation and you have suggested one.
This is why these are called guidelines. Often, there is NOT a specific provision and that is why this is taking so long.
The issue here isn't so much the actual photo credits, but rather the issue of including or omitting text. The trend has moved steadily towards following print and NOT taking it upon ourselves to omit what is printed.
List photo credits in contents because we list everything printed in contents. If you are omitting any photos, explain in a TN. If you are omitting photo credits, explain that in a TN as well. Photo credits that don't match up with a picture can be omitted. Use your judgement. Depending on the situation and IF the credits are listed in contents, you can say that credits for omitted pictures are also omitted.
Pictures without captions--you may decide to describe some or all of these. You certainly should if the text refers to them in any way. But pictures with no captions that do not add information to the text can be omitted with notice. See 6.1.2 about omitted pictures with no caption.
Pictures that do have captions are generallly included along with accompanying credits that are present, usually following the caption.
For pictures that have ONLY the credit, consider treating that credit as a caption and describing the picture in a TN. If that picture truly should be omitted, consider a TN that notifies of the omission at that point and briefly mentions the credit by way of identification.
I hope this helps. That's why they call these guidelines.