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 5 feet) 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 spell takes effect after?
If you're attacked first, then sanctuary activates. Assume the creature fails its save, because if it passes, the question is moot. The only available target is your mount, which it then chooses to attack. Mounted Combatant can then be used to force the attack to target you again. If the creature fails again, the attack is wasted, as the attack is forced to target you instead of your mount.
If your mount is attacked first, you can use Mounted Combatant to force the attack to target you instead. Sanctuary then activates, and the creature fails. It's unable to attack the mount again, because you've forced it to target you instead, but it's also unable to target you, so it's attack is wasted.
In both cases, after you've used Mounted Combatant, the mount is not a valid target, because the feat "force[s] an attack [...] to target you instead". Since this is the same attack, and it's been forced to not attack the mount because of this feat, it cannot be re-targeted, even after sanctuary applies.
Sanctuary says (emphasis mine):
Until the spell ends, any creature who targets the warded creature with an attack or a harmful spell must first make a Wisdom saving throw. On a failed save, the creature must choose a new target or lose the attack or spell.
The consequence that a failed save against sanctuary requires the selection of a new target for the attack means that the saving throw can be attempted at most twice (in this case, of two creatures in reach of the attack). Regardless of who is attacked in what order, at the point in time when the attack has already targeted both the monk and his mount, there are no new targets available and the attack is automatically lost. Cf. If 2 creatures in range have the Sanctuary spell cast on them, can you repeatedly switch targets between them until you succeed on the save?
If the monk is attacked first, and the save is failed, the spell forces the attacker to either select a new target (the mount) or lose their attack. Targeting the mount will trigger the Mounted Combatant Feat, permitting the monk to force the attack on the mount to target1 them instead. When the monk is again targeted, a second failed save means the attacker must either choose a new target or lose the attack, but now there are no targets that have not already been subjected to the attack, so the attack must be lost.
If the mount is attacked first, the attack triggers the Mounted Combatant Feat, permitting the monk to force the attack on the mount to target1 themselves instead. When the monk becomes the target, a failed save means the attacker must either choose a new target or lose the attack, but now there are no targets that have not already been subjected to the attack, so the attack must be lost.
1 Trolleitor asks whether the monk must save against the sanctuary spell in order to use the Mounted Combatant feat. Sanctuary says "any creature who targets the warded creature with an attack or a harmful spell must first make a Wisdom saving throw". It is not clear whether this means [any creature who choses the warded creature as the target of their own attack] or [any creature who selects the warded creature as the target of any attack (including the attack of another creature)], and this is perhaps worth its own question. However, consider the consequence of a failed save; the creature attempting to target the warded creature must select a new target or "lose the attack or spell". It is not clear to me how I can "lose" an attack I am not myself making, even if I am permitted to select its target. Thus, personally, I would go with the first interpretation and not require the monk to save against sanctuary. If the DM favors the second interpretation, however, then the sanctuary spell could indeed force a third save, with the maximum then being two from the attacker and one from the monk.
If you force the creature to target the monk, he will trigger the condition for sanctuary again, so the creature will make another saving throw. Just like if it made another attack against the monk with multiattack.
Nothing on sanctuary prevents a creature targeting the monk after a failing saving throw in the same turn; the only trigger condition is "targets the warded creature with an attack or harmful spell". Also, the creature can't lose the attack, because the creature chose a new a target.
The spell will follow this sequence: Triggered by the attack > Roll saving throw (failed) > Choose new target or lose attack (choose the mount). At this point, sanctuary ends for that creature, the attack continues unaffected, and a new trigger happens (an attack targeted my mount) with the sequence: Triggered by the attack > Target the monk instead ... And repeat.
Because there is no limitation on how many times sanctuary or Mounted Combatant can be used on a turn, the order is also irrelevant.
