A Celestial Hybrid is like most Celestials in a lot of ways. They have Authority and need faith to create divine energy and advance their domains except that Celestial Hybrids have a unique divine energy. The tree father has the divine life energy that is strong enough to resurrect a mortal and create transcendents from ordinary trees.
The tree father can also spread its seeds as a life tree does. The original life tree was the body of the high elf ancestor. The other avatars are from its seeds. The seeds of the tree father grow into his avatar instead of new life trees because Life trees can create seeds and gods can have avatars. The fusion of these two abilities brought about the avatars.
All of these confused the will of the divine plane of the Zargoth when it sensed the avatar. Gods aren't supposed to be able to move out of their plane either in their main body or through avatars unless they are attending the trial of heaven. The tree father's avatar broke that normal condition. It helped the tree father to move around since it can't leave the inner world of the world beast.
The green elf can't leave the inner world. It is bound to the inner world and has become one with it so it cannot leave it but its seeds can. The weirdness of the situation made the divine will of the Zargoth plane unable to determine what exactly the Avatar is. That helped to give the tree father's avatar time to prepare or Zernon will have come after it immediately after he sensed a new Celestial.
If the avatar had been fully recognized as a Celestial then the events that made the Zargoth plane fall under the control of Legion wouldn't have happened. Zernon would have sent his inquisitors and punishers. They would have attacked the giant tree from every direction.
Instead of a heroic quest, Zernon would have called for a world war. The avatar would be at a disadvantage since it can't move and it would have eventually been destroyed despite its ability to create transcendents. The avatar of tree father can't face off against the entire pantheon so it will be doomed since it can't run away.
The tree father would have survived at the end of the day. They can kill his avatar but they can't stop him from trying again. He can also send more than one avatar to the plane and divide their forces. The only constraint that the tree father faces to the production of avatar is Origin essence. He also needs divine energy to produce divine life energy. But those things can be gotten in time and in time the tree father will eventually ground down the resistance it will face.
It will be a bloodbath but the tree father will eventually infiltrate the Zargoth plane. It is just that Legion would have lost the element of surprise which will hamper their plans severely. For one, the church of Life won't be able to grow and flourish in the short time it did due to the persecution that the believers will be facing. It will get worse if the tree father is labeled an evil god. It will make it very difficult for the church of life to take root within the plane.
The lack of secrecy will also make Zernon take any anomaly seriously. He will never allow Helios to open the portal to the abyss. He will use everything he has very early including the GodSlayer. It might also make friends with the god of fate to protect the plane. An entirely different story would have played out had the presence of the tree father been known right from the start.
It is not pure speculation because the invasion of planes is not an isolated situation. Zargoth plane is not the only plane that Legion-1 spread the seeds of Legion-9 to. The tree father faces different kinds and varying levels of opposition. Some Celestial Supremes kill other Celestials for various reasons. It could be because of power or the need to secure their seat of power.
Some other planes are divided into different warring pantheons. They are not united like the Zargoth plane and they don't all follow one leader. There are several factions with several Celestials in the plane. Some of the factions will gladly accept a new celestial into their fold, unlike others that will kill any new celestials so that their religion won't have much competition.
But most planes were hostile to the tree father. They have a problem dealing with the fact that he just sprang up from nowhere. Something about his lack of a past makes other gods suspicious of him. It caused the tree father to be discriminated against and viewed as an outsider.
There's also the problem that he can't move. He is in the mortal realm and he can't escape. The avatars are not hidden in a divine kingdom. They are in a forest that is easily accessible to everyone. That's why his forest in other planes became the destination for heroes and demigods that want to kill him for his Godhood.
The tree father has seen a lot of things through its avatars and has made some progress in the other planes but it is only from the Zargoth plane that one of its avatars can become the only god and horde all the faith to itself. It is not a large amount because of the small population of giants of order and the other races but it is a monopoly nonetheless. The population will also increase now that mana is saturating the plane.
Even more important than the sole godship of the plane is the Authority of the Celestial Supreme. Zargoth plane is the only plane that the tree father has a shot at becoming the Celestial Supreme. The help the avatar received from Helios and Aeternus allowed it to achieve everything it did in the plane.