[RPG] Can the Help action be used to give advantage to a specific ally’s attack (rather than just the next ally who attacks the target)

If Aiden uses the Help action to give someone advantage when attacking an enemy, that is the only effect that Help action has; it doesn't give any ally advantage on an ability check, since Aiden already decided to use it to gain advantage in attacking a creature.

However, according to Jeremy Crawford, the Help action does not need to specify a particular ally to gain the benefit:

If you use the attack-aiding option in Help, the next ally who attacks the target gets the benefit.

So, in your example, Aiden would use the Help action to assist a friendly creature (any single one) in attacking the specified enemy orc (which must be within 5 feet of him when he uses the Help action to grant advantage on an attack). The orc would then reduce Charles to 0 HP.

However, when Arthur attacks the orc, he will have advantage (assuming there's nothing else that would give him disadvantage), because that's what the Help action does; it distracts an enemy in order to give an ally (any ally) a better chance to hit.

Yes, familiar can be used for gaining advantage for allies.

From Find Familiar spell description (PHB, p240), emphasis mine:

Your familiar acts independently of you, but it always obeys your commands. In combat, it rolls its own initiative and acts on its own turn. A familiar can't attack, but it can take other actions as normal.

Help is listed as one of the actions that could be taken in combat and is a legitimate option for a familiar.

And it's also logical thing from roleplay point of view. From Help action description:

Alternatively, you can aid a friendly creature in attacking a creature within 5 feet of you. You feint, distract the target, or in some other way team up to make your ally's attack more effective. if your ally attacks the target before your next turn, the first attack roll is made with advantage.

As you pointed out, even small familiars can distract enemy, which will help ally in their attack.

The Word of Designer also supports this. Jeremy Crawford answered this question in the September 2016 Sage Advice column:

Can the familiar you conjure with the find familiar spell use the Help action to grant you advantage on your attack roll? A familiar can’t attack, but it can take non-attack actions, including Help. As the text of the Help action indicates (PH, 192), the action doesn’t require you to be able to attack; you simply need to be able to provide some sort of distraction.