As Jim rightfully said, there is no need to do more than removing old oil/grease (if there is any), and stabilizing active rust.
For leather conservation, I actually recommend neatsfoot oil, as a lot of commercially available leather conditioners contains mineral oil that may be detrimental to the conservation of leather on the long term.
For the shagreen, I can't really help as I have absolutely no knowledge or experience related to its care, and I can only recommend that you be very careful and read extensively on the subject before attempting anything, as I do know it is very delicate.
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