Great summary!

It seems that we will never get a SBD percentage greater than 10%, So sbd_print_rate will never be zero?

Interestingly, the sbd_print_rate seems to be calculated from the virtual supply here, while the median price is defined from the "actual" STEEM supply as you linked. With the SBD debt at 10% at most and the virtual supply always greater than the "actual" supply I think you're right that a 0% print rate can never be reached. Is this intended, or am I missing something?!

edit: Even though the median price is calculated with the actual supply, the arithmetic is chosen in a way to fix the SBD debt wrt to the virtual supply.