Salat al-Istikharah (Arabic: صلاة الاستخارة) is a prayer recited by Muslims when in need of guidance on an issue in their life. The salat is a two raka'ah salat performed to completion followed by the supplications Salat al-Istikharah.
The Istikhara done by a person does not bind other person even if they are husband and wife unless it was done by request from the husband and his wife was just doing it on his behalf and on his intention.
To answer your question, yes it is possible for a dua to change a given situation. With that said, i would caution against overuse of Istikharas. They should only be performed after one has utilized their intellect to the full extent, and they have consulted with others, and done everything within their reach to reach a solid conclusion. If they still haven't been able to reach a conclusion, only then should an Istikhara be utilized.
In some cultures in the Middle East, Istikharas are over utilized while God has given us an intellect for a reason....
Istekhara can not be repeated on the same matter unless the circumstances change. Du'a can help in changing the circumstances then another Istekhara can be seen.
The second Istekhara is invalid because Istikhara can not be repeated unless and until circumstances change.
It depends on the verses. Most results can be interpreted as "yes" or "no". Occasionally it does not give a strong view either way. You could ask someone for a second opinion on it. Generally the result of an istikhara does not come in the form of a command but rather is interpreted from the overall content and context of the verses.
Istekhara from the Holy Qur'an has many conditions, without fulfilling
all of which, no one will be able to do it and understand it properly.
Among the conditions is the ability to understand the meaning of the
Qur'anic verses. Not only the verse which comes on the top of the page
but also the meaning of the verse before it and after it.
Many of the leading Ulema advise to seek Istekhara of Tasbeeh rather than of the Holy Qur'an except for those who really have the all its conditions
met. Some people refer to the translation to understand the meaning
which is not accurate always, and any sort of less understanding of the Holy
Qur'an can give inappropriate understanding of the result of the
Istekhara. There is no difference between the Istekhara of Tasbeeh and
the Holy Qur'an because we are seeking guidance from the same source
who is Allah (SWT). So there is no point in thinking that the Istekhara
from the Holy Qur'an is more appropriate especially when the
conditions of the Istekhara of Holy Qur'an are not fully met.
Yes (as long as there are no reasons why they are forbidden for marriage, for instance they are mahram).
Istikhara is never required. One can decide to perform it, but since it is not an obligation, the answer to your question is no.
Please have a look at an interesting lecture about Istikhara in general:
Hope this helps out understanding the matter inshaAllah.
The istikhara is a complex issue which must be fully and carefully understood, however in terms of your question, there is no requirement in Islam to "perform an Istikhara" for marriage. One should use their intellect and investigate about the potential spouse through the various permissible means before deciding to marry someone.