Since the question on whether or not a sprite can remain invisible and use its help action has already been answered, I'll jump to the one bit that hasn't been answered so far:
"My only concern is: is such help strong enough to provide me with advantage?"
Yes you get advantage
The Help action itself is quite remarkable (and underused in my experience): you give up your attack - and thus your ability to do damage yourself - in order to give one of your allies advantage on its next attack on one specific creature you choose.
You give up your Sprite's damage to the foe and in return you get a greater chance to hit, and thus have a bigger chance of doing damage yourself.
The way your sprite's Help action plays out is mostly flavour. What is does, however, is determined by the Help action's definition, which you've already found.
My answer to your question would be: Yes the Sprite would grant you advantage.
But...
You have already stated you're aware of the fact that creatures could sense the fact that your sprite is standing/flying next to them, when you wrote: "(but of course can still be sensed by some creatures, or heard by them)"
Keep in mind that your Sprite has to be within five feet of the enemy in order to take the Help action and can still be attacked while invisible, although this means the attacker will do so with disadvantage, due to the way the Invisible condition is described:
Invisible
An invisible creature is impossible to see without the aid of magic or a Special sense. For the purpose of Hiding, the creature is heavily obscured. The creature’s location can be detected by any noise it makes or any tracks it leaves.
Attack rolls against the creature have disadvantage, and the creature’s Attack rolls have advantage.
If your sprite were to stay in the enemy's reach, this means they could attack it on their turn, but with disadvantage. Your Sprite can, however, fly out of the enemy's reach without provoking an attack of opportunity, assuming it has any movement left.
RAW-wise, an attack of opportunity can only be made against a creature the enemy can see, which gives your Sprite the possibility to aid and move out of the enemy's reach without taking any damage and without requiring the owl's "Flyby" feature.
Edit: I found out this question has already been answered, for more elaborate explanations, go here.
Yes, the familiar can grant advantage with the Help action, and its presence adjacent to an enemy can allow sneak attacks. Bear in mind, though, that most familiars have extremely low health and AC, and will likely die in a single hit, requiring 10g and at least 1 hour to resummon.
Best Answer
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:
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.