I have not completed any derivations in a while. I have attached the transfer function derivation of the INA121 from Texas Instruments using Superposition.
This will be the next subject to put on my website.
Please review for any errors.

 

Looks good to me. Here's what I got using another method:

 

Your arithmetic is fine Sir Knight.
Thought you might be interested in adding this derivation of the statements you have made in square boxes, without proof. I have followed your notation.