Are imaginary or unintentionally uttered divorces valid, and what is the ruling on phrases spoken in the context of marital disputes such as "khalas" (that's it/it's over) and "we'll live together like sisters"?

The word "khalas" (enough/finished) is not an explicit pronouncement of divorce, so no divorce occurs without the intention thereof. Since you did not intend divorce with it, or you doubted the intention, it does not count as a divorce. Doubt concerning an oath of divorce does not make it binding. And your statement to your wife, "We will live together like siblings," is not a divorce. Therefore, do not pay attention to these doubts.