For this scenario, assume a level 11 Monk with the Mounted Combatant feat (and riding a mount).

If a creatures attacks the Monk, and the attacker fails his/her save, the next eligible target (assuming no other enemies within 5ft) would be the Monk's mount. If the Monk decides to use Mounted Combatant to force the attack to target himself instead of the mount what happens?

Does the situation change if the Mount is targeted first, then the Monk's Sanctuary takes effect after?