The strength of the ban on his name is more powerful than his order for space to freeze in the mortal realm because it has a localized effect. Instead of all his power spreading throughout the mortal realm, it focuses on whoever is trying to utter, write, or communicate his name through any means. Only a Celestial god can bypass that ban. So he was surprised that a god knows it when that god should be a new god. It made him worry about how powerful the hidden god really is. He was also chilled when he saw how that god descended onto that transcendent to utter his name.
A god can use others as their avatars only if they are an accommodating vessel for that god. The best accommodating vessels are ones that have been bestowed divinity. But mana entities are too weak to be useful as vessels while transcendents absorb divinity very well and wouldn't become a vessel for a god.
