You asked: Is The Urantia Book infallible?
As far as the cosmology of The Urantia Book, we can say that the information contained is not infallible, by the admission of the revelators themselves, as in this section. However in that section we also read:
101:4.3 Truth is always a revelation: autorevelation when it emerges as a result of the work of the indwelling Adjuster; epochal revelation when it is presented by the function of some other celestial agency, group, or personality.
101:4.4 In the last analysis, religion is to be judged by its fruits, according to the manner and the extent to which it exhibits its own inherent and divine excellence.
The Urantia Book is a book of information about a great number of things, but primary among those things is its teachings about religion and its revelations about God. Its teachings present only God as infallible, the indwelling adjuster as infallible; nowhere in the book is a claim made that it, in itself, is infallible. Nevertheless, the truth that it contains can lead to a feeling that one has found a source of information that can be relied upon. As the book says, truth can be acted out; it can be lived. And it is in this understanding that we find assurance that what we are reading is true.
In the end, it’s not the book as much as what the book inspires in the heart of the reader that is important.
Truth is always dynamic and moving; always relative to circumstances. But truth is also always recognizable to one who is a truth-seeker. This is one of the functions of the Spirit of Truth: to enable seekers to discern the truth wherever they may find it. And many people discover truth in the teachings of The Urantia Book.
2:7.6 (42.7) Intellectual self-consciousness can discover the beauty of truth, its spiritual quality, not only by the philosophic consistency of its concepts, but more certainly and surely by the unerring response of the ever-present Spirit of Truth.
The revelators of The Urantia Book warn against making any object into a fetish. In some ways, this has been done as regards the Bible in that it is thought by many to be infallible – that every word is true and that dire consequences will attend anyone who deviates from that belief. But this is not so with The Urantia Book. Its teachings point believers in the direction of God alone, to the presence of the indwelling Adjuster alone, as the source of truth. And these spiritual forces can be relied on as our ultimate authority of truth, along with the assistance of the Spirit of Truth and the Holy Spirit.
I hope that this reply is helpful.Thanks so much for writing.