Since I am family sharing the first two DLCs (Warmind and Osiris), I do not have access to preorder forsaken on the Xbox store. If I order the DLC via gamestop and receive the code to redeem, can I redeem the DLC on my account that does not have the first two DLCs? TL;DR: Can I redeem and have access to Forsaken if given a code from gamestop despite not really owning the first 2 DLCs (family sharing from friend)?
You will not. The code you get from GameStop is just for digital currency for your platform store. You will not be able to purchase Foresaken with this because CoO and Warmind are required to be owned by the purchasing account.