Gareth just say something like:
"I appreciate the offer of the mouse, but I feel that it wouldn't be of much benefit to me due to the button layout, and the fact that I still wouldn't be able to map the keys for movement to the mouse. I also feel it would be no benefit to others in the same position as myself, which I also hope me speaking out could benefit too.
All I need is some way in the game for me to map my mouse buttons properly and this would allow me to play and enjoy the game I paid for. Please don't think I'm not thankful for the offer of the mouse, because I am, all I ask is for some support for binding mouse keys in the game, which could be patched in sometime in the future to help not only myself, but anyone else in a similar position to me."