Larvae (and the division thereof)

Or maybe, just to say, the nalfeshnee keep an abomination imprisoned in the mountain, and they know how o free him.

The fear of the nalfeshnee freeing him for revenge could prevent the demon lords fro trying to conquer the mountain of woe. 

Demogorgon is powerful, but as I recall he also spends a tremendous amount of time in his own keep arguing with himself. Graz'zt potentially could, but I am not convinced that he would. Certainly, he controls three layers right now, but that does not mean he can take and hold a fourth. As mentioned, the Abyss holds tremendous politics. Furthermore, Graz'zt's layers are all not only directly in order, but so intermingling as to almost be one single layer instead of three separate layers.

It's been a considerably long time since I've read Faces of Evil, but consider this for distribution of larva. Suppose the night hags have discovered a way to get their grimy fingers into the larva trade on the Abyss. They certainly trade larvae in other areas and I do not doubt they'd love to get involved in Abyssal larvae, as well. Suppose one hag or a small coven of hags got ambitious and started manipulating the flow of larvae? It seems a very haggish thing to do and it would give them a foothold in the Abyss as well as a bit of leverage against the Princes and Lords.

It is important to remember that there is a hierarchy in the Abyss. It is a difficult hierarchy to understand when put in the context of "chaos = complete and utter disorder" but the hierarchy exists. Each type of tanar'ri has a purpose in its existence. There is a role it plays in the grand scheme of things. Glabrezu and succubi are tempters, mariliths are tacticians, etc. There's also a serious game of power balance and politics in the Abyss. The nalfeshnee might work together because it serves their individual purposes better. Consider that they want to control this plane, but they know if they turn against each other, then they'll ALL lose. That could be unacceptable, so they work together to keep hold of their position.

It could also very well be that the other Abyssal lords are just fine with letting someone else handle this.

Moreover, there are a few things that seem bizarre, but still exist. For example, Red Shroud is a succubus who controls the city on the Plain of Infinite Portals that holds the gate to Plague-Mort. Why? How? Succubi aren't greater or true tanar'ri, and they're barely strong enough to stand up physically to most fiends, so how does she manage when most creatures on the Abyss could rip her apart?

That being said, it's shown that tanar'ri can cooperate. The Blood War, 'loth-driven as it may be, is the greatest example of this. When driven toward a common goal or purpose, the tanar'ri are capable of banding together. There is an understood order and system in the Abyss, even if the place is a den of destruction. It is not nearly as clear or structured as Baator's, but it exists nonetheless.

In the end, though, you as the DM are perfectly welcome to "reject reality and substitute your own," so to speak Merging the five nalfeshnee into one demon lord seems a perfectly reasonable idea. You might set it up so this demon lord is a puppet of the hags. If one hag can overthrow and become a Lord of the Nine, certainly another could exert control over a tanar'ri lord. Might be why he's so fat, too -- the hags are feeding him larvae (and whatever else) to make him complacent and sedentary while they really call the shots. You could even build a story arc around other Abyssal lords discovering the hag interlopers, and then get PCs caught up in the fallout as one of the lords tries to unseat the fat guy and chase the hags off. Alternatively you could do a very similar storyline by using a rilmani interloper instead of hags. Or a rilmani pretending to be a hag.