-edit- added the savescum savefile if anyone wants to try.
used your savefile, could not win the race with your slave but I did win it with your assistant xD here are the steps:
-save starts in white town so immediately go to trade sqare, buy wedding dress, go home
-swap slave with assistant, equip her with the wedding dress, plumed briddle, ornamented collar, pony harness, hooved boots
-set your slave (who is now your assistant) to wash the slave, then wash her
-encourage her once, use sententia veritas spell
-train pony once, after that reward with sensual massage
-smell pollen, tell the girl to smell pollen, make her drink kamra
-train pony until S (should end up with 3 positive energy which turned purple)
-go racing, take heavy wheels and armored sides
-use the following commands: start -> whip -> whip -> whip -> manage -> draw -> draw -> draw -> whip -> manage -> whip -> whip
kinda cheated but hey, a win is a win, right?