Does swallowing saliva carrying the taste of juice, which was drunk before wudu, invalidate the Fajr prayer, especially if the worshiper felt that he had exited the prayer—or that his prayer was invalid—during bowing, and then completed his prayer?

It appears the questioner is afflicted with waswasah (obsessive whispering/doubts). The presence of juice taste and swallowing it with saliva does not harm the prayer, nor does it affect its validity. Similarly, her feeling that the prayer is invalidated without a real reason does not affect its validity, because it is merely waswasah. As long as she did not intend to cut off the prayer and did not confirm the existence of something that invalidates it, her prayer is valid and she is not required to repeat it.