Hello! Yes, this is an old thread but I'm inspired to say a thing or two

1. Is there a possibility that the authors of the UB be untruthful or at least deliberately hiding information?

I believe that we are clearly told that one of the authors' prime goals or mandates is to correct current human understanding, especially concerning cosmology. However, the authors are limited to present primarily "earned" knowledge and not present unknown facts that should be divulged in more distant times of human development. The general preference of the authors, it appears, is to use already appearing human sources as the basis of the narrative and modify the expression only as much as is needed to correct erroneous ideas. Therefore, their "truthfulness" might be a bit relative and context comforming, in each of our own individual evaluations.

2. Are there higher beings than our local universe Father in the totality of existence?

There cannot be. We are told that the Universal Father is, along with the other 2 members of the Trinity, absolute, total, perfect, infinite in all ways and is the source of all things in time and eternity. Some of those aspects are talked about in terms of "Buddhas" in Buddhist literature, but not all, or is there something more to consider there? Remember too, that the UB authors speak of hosts and hosts of transcendent or eventuated beings that have no beginning and end (in time) and do partake of aspects of infinity, however, they are classified as creatures rather the creators. Then too, there are the experiential deities such as the Supreme Being.

3. In my religion, there is a section called Abhidhamma (a book about the minute details of human mind). In my opinion, the content there is superhuman. What do you think about this?

There surely exists a bit of both Hinduism and Buddhism that is derived from superhuman knowledge. The ancient centers of civilization and development on the Earth apparently sometimes had access to superhuman knowledge as we humans are not the only inhabitants of this planet. I believe the authors of the UB had no intention to attempt to state all possible knowledge, but rather, primarily the things that matter most to all of us collectively and which could change things for the better.

P.S. Is it the case that not all Buddhists are aware of and understand the concept of Maitreya Buddha? The UB describes Machiventa Melchizidek who functioned very much as one incarnation of Maitreya Buddha. We are told of future bestowals of superhuman teachers, even comparatively divine personalities, who are so much more spiritual that we as humans cannot even see their kind. However, they function as teachers extremely effectively just the same.