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.
It's almost correct, but not quite.
There are two separate, unrelated ways to get a familiar in D&D 5e, and you're trying to combine them.
The first method is by use of the Find Familiar spell, which allows you to summon a celestial, fey, or fiendish spirit that takes the form of any of a list of creatures. This list is expanded by the warlock pact of the chain. This familiar is perfectly obedient, can be resummoned when it dies, can be hid in a pocket dimension, deliver touch spells that you cast, and everything else specified in the spell description. This method gives you a familiar with the basic stats of the chosen creature, not the "variant: familiar" traits of the chosen creature (unless your DM chooses to have that creature type appear).
The second method is by finding a quasit, imp, or pseudodragon that has the "variant: familiar" trait (which is 100% up to the DM), and enlisting it as a familiar by interacting with it. This familiar has only the traits listed in the stat block for that creature, including the variant traits, but none of the traits of familiars given by the Find Familiar spell. No pocket dimension, no touch spells, no limitations on what actions it can perform, and if it dies, it's just dead. This type of familiar is an NPC controlled by the DM (much like a hireling or other follower), and is only as obedient to the PC as the DM says it is, using the MM entry as a guide.
Now that we've established how things actually work, we can address your real concern. Your warlock can't yet communicate at a great distance, but he can soon. There is a warlock invocation available to him called Voice of the Chain Master that does the same thing, but with unlimited range on the same plane. As you've realized, this ability has some incredible potential, especially for scouting.
It's not any more powerful than other options and should not be limited.
Let's compare it to some other invocations. There's one that lets a warlock cast Disguise Self as at will. This would let him see a guard, and appear exactly like that guard and just walk around the enemy camp unimpeded. Or maybe impersonate the leader of the camp and just take it over without even a struggle. There's another invocation that lets the warlock cast arcane eye at will, which gives you a way better scout than an easily killed creature. A familiar, even an invisible one, still has to succeed on a Dexterity (stealth) check to avoid being heard and then easily killed. An arcane eye does not.
So, in order to fully utilize this scouting ability, your warlock has to pick the chain pact and spend one of his few, precious invocations, both of which are huge opportunity costs. He deserves something in return. This something is you not limiting it. It already has a flaw in still being able to be killed by anything that hears it, or smells it. That's right, just about any pet wolf is going to be enough to catch this familiar. It doesn't need any more limitations.
Best Answer
Two points suggesting that the Sprite isn't a poison factory
The language you cited is general enough to suggest that this ingredient collection, and subsequent poison fabrication, is a collective activity undertaken by Sprite communities in the areas where they live. "In their forest domains" or "they venture" .. is the third person plural which can be read as it being done in groups.
The spell that summons a Familiar summons a spirit that takes on the form of a ____________. (Usually an animal, ref is PHB p. 240). This is not modified by any language in the Pact of Chain class skills description (PHB p. 107). This means that your familiar Sprite is a spirit in the form of a Sprite.
Ask Your DM
Your DM might be open to a limited capacity for such production, per cost and time guidelines you mentioned, if for example you agreed to go without the familiar for X period of time so that it could get the job done. That's a "DM ruling" point and not a "what's written in the book" point.