1- Istekhara would be appropriate when you are hesitant and stuck. You contemplate the decision, you research and you consult others, but you are still stuck. In this situation it would be good to take an istekhara.
2- We cannot take istekhara for other’s people’s decisions—unless they ask us to and make the intention. So you cannot take an istekhara with the intention “Should my son marry this girl” without your son knowing and making the intention. Yes, what you can do is make the intention like this: should I encourage or push my son to marry this girl. This would be valid.
3- Both forms are valid (Qur’an and Tasbih). So it’s up to you. Scholars generally prefer to do it with the Qur’an on important matters.
4- If the istekhara turns out good you don’t have to follow it. But if it turns out bad, it’s not haram to not follow it, but it’s not good. One will regret it or bad things can happen.