How do the Mounted Combatant and Mobile feats interact with opportunity attacks?

Question

In short, when a Mounted Combatant & Mobile feat user redirects an attack from their mount, does that attack fail?

Obviously, all the conditions that allow a mobile user to avoid opportunity attacks would need to be met.

One of the benefits of the Mounted Combatant feat (PHB, p. 168) is:

You can force an attack targeted at your mount to target you instead.

One of the benefits of the Mobile feat (PHB, p. 168) is:

When you make a melee attack against a creature, you don't provoke opportunity attacks from that creature for the rest of the turn, whether you hit or not.

Example:
Our rogue horseman name C.C. (who has Mounted Combatant & Mobile Feats) rides up on the belligerent troll and horsewhips it across the face, then rides off. The last 3 times the troll tried to retaliate from CC's ride-by whipping, it was stopped because of the Mobile feat. Since this is an above-average-Intelligence troll, this time it decides to attack CC's horse. But oops, when the troll does that, CC says that he will just redirect the attack against the horse to him instead.

Now what happens?

Since CC nobly threw himself into danger to defend his horse, does he take the hit?

Or is CC so dodgy that redirecting the attack allows him to use the Mobile feat benefit to avoid the redirected attack?

Does it matter if CC is controlling the horse, and does that mean that CC provoked the opportunity attack?

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Related: Who gets hit by a melee attack when you have Sanctuary and Mounted Combatant?

Answer 1

CC takes the hit.

The horse is the one provoking the attack of opportunity, no matter who the attack eventually targets.
After the attack of opportunity has been provoked, the troll is simply making a melee attack against a target - and the Mobile feat does not provide you immunity to being targeted by melee attacks.

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Some context about opportunity attacks, mounted combat, and the mobile feat:
When riding a mount, you are moved by someone without using your movement, action, or reaction, and as such you do not provoke an opportunity attack.

You also don’t provoke an opportunity attack when you teleport or when someone or something moves you without using your movement, action, or reaction. (Chapter 9, Making an Attack - PHB)

Mounted combat also specifies that an attacker can choose to target you with an opportunity attack even when you don't provoke one, if your mount provokes an opportunity attack.

if the mount provokes an opportunity attack while you’re on it, the attacker can target you or the mount. (Chapter 9, Mounted Combat - PHB)

So in actuality, the Mobile Feat provides no real benefits while mounted:
You are using your mount's speed to move, so the speed increase does not matter;
Your mount would be the one taking the dash action, not you;
You already do not provoke opportunity attacks when mounted.

And aside from that, the discerning rider might want to have his mount Disengage as its action, avoiding this situation entirely.