What is the ruling regarding one who doubts the number of disputed matters and his intention concerning divorce, or regarding what he swore an oath upon; can he perform Istikhara or Salat al-Hajah to resolve the doubt?

There is no room for the Istikhara prayer in such matters. However, one can supplicate to Allah for guidance and for the truth and correct path to be made clear. Then, it is obligatory to ask trustworthy people of knowledge, in compliance with the words of Allah Almighty: "So ask the people of the message if you do not know." (Quran 16:43) Regarding what is doubted, "certainty is not removed by doubt," and the meaning of this rule is that things remain as they are until the contrary is certain, and an incidental doubt does not harm it. Accordingly, doubting the intention of divorce or its occurrence does not affect the marriage contract, and divorce does not take place by it.