What is the difference between Istikhara and Khira, and does asking shaykhs about the khira of marriage fall under the prohibition of asking soothsayers? And what is the correct interpretation of Istikhara, especially if matters become complicated after embarking upon them?

When a servant is about to undertake any matter, they must seek guidance from Allah (Istikhara) and consult with experienced individuals.

If the shaykhs advise based on circumstantial evidence and observable facts, such as the situation of a young man proposing to a girl, then there is no harm in that. However, if they claim knowledge of the unseen, this is an act of soothsayers and diviners, and it exposes those who approach them to a severe punishment, as Allah Almighty says: ﴿Say, "None in the heavens and earth knows the unseen except Allah. And they perceive not when they will be resurrected."﴾ (An-Naml: 65).

In this case, their advice should not be heeded. Rather, the servant must seek guidance from Allah (Istikhara) and entrust their affairs to Him. If the matter is facilitated, then it is good. If it is not facilitated, then one should know that there is good in its absence, and they must praise Allah and be content with His decree.