Does the fiancé's request for his fiancée to photograph her constitute a reason that would necessitate a bloodwit or expiation upon him after her death in an accident, knowing that she was holding the phone at the time of the accident?

You are not held responsible for her death, nor is any blood money or expiation due from you, even if you had asked her to photograph her clothes or agreed with her to photograph them, as this is not the cause of her death. If you were engaged but the marriage contract had not been concluded, then you erred in your interaction with her as a non-mahram (definition).