Idea 1: Common Enemy
A powerful and influential group of witches will become complacent and fight amongst themselves for power. To prevent this you should make your witches less powerful.
Better yet give them a common enemy to unite against. And for witches there is no better enemy than the establishment.
Witches are feared and hated by the common folk and persecuted by the establishment. Hundreds of witches are drowned, crucified, and/or burned alive each year. The number triples if you include suspected witches and witch sympathizers.
The witches are too busy hiding and fighting the establishment to compete amongst themselves. Their shared enemy creates a feeling of solidarity among witches in separate clans on opposite sides of the country who have never met in person.
There are still arguments over tactics and over any shared finite resource. But the arguments center around how to best use the Place of Power for the common good, rather than fighting over individual access.
Idea 2: Witches are Nice to Each Other
You don't get very far as a Witch without being an emotionally sensitive and sympathetic person. This is the difference between Witchcraft and Sorcery.
Sympathetic people have the skill to unconsciously adapt their emotional state to blend in with their surroundings. it turns out the skill is similar to modulating your internal mana so it can resonate with the ambient mana in the area.
This is a recurring theme in Global Witchcraft. There are many Prophecies about the Witch Messiah reincarnating to save us all. In fact many of these prophecies have already come to pass. The problem is that when the Witch Messiah finally appeared she was too powerful -- and too sympathetic -- to make any use of her powers at all. She viewed the entire planet as her extended family and did not wish to harm either the covens or the establishment.
For us mere mortals, the result is that every Senior Witch views all witches in the country as family members and does not play favourites. This leads to a Witch's Council full of nice people who are sympathetic to each others' needs. They do not do infighting.