But given the characteristics of it what I wrote in the first post, would the diode setup work?
Your diode steering circuit looks like it would work to me. I would use diodes with as high a current rating as you can find (and afford of course). As a second level safety item I would consider adding one automotive style fuse on the single common - lead feeding the motor. Who knows what the stall current would be if the motor was to jam or whatever and while the motor driver seems to have some fuse protection I would consider this second one maybe sized closer (but still more) to what the motor actually draws in your application. Note that because of the voltage drop (across two diodes) there is likely going to be a little 'dead band' around the neutral control speed setting being sent to the controller, but you can probably deal with that in your software if it becomes as issue in your control algorithm.
Lefty