Is uttering untrue words during daydreams considered a lie, and what is the ruling on that if it includes an oath that cannot be recalled, and is it considered backbiting to utter something offensive about a person without anyone hearing it?

It is recommended to turn to Allah through remembrance (dhikr) and prayer when feeling distressed and troubled, for indeed, by the remembrance of Allah do hearts find tranquility.

As for what occurs by way of speech in solitude, it is not considered a lie nor backbiting, nor does it entail an expiation for an oath (kaffarat yameen) or the like, because it falls under the category of a 'vain oath' (laghw al-yameen) for which Allah does not hold one accountable, especially if there was no intention to make an oath and doubt exists regarding whether an oath was even uttered. The fundamental principle is the acquittal of one's liability.