Prayer

You have to pray Fajr on the plane and you can’t postpone it until sunrise. Therefore, you should observe the break of dawn and pray it before sunrise.

This is true if you are traveling from your hometown to another city. If this is the case, you can shorten prayers and combine them at the time of Thuhur or Asr up to your convenient.

Your friend’s method is safer though what you had done is accepted as well.

There is no limited number as long one doesn’t pray: a. After Fajr b. After Asr c. Between Athan and Iqamah of Fajr

Yes.

The minimum is 2 and there is no maximum

It is an issue of dispute among scholars. There isn’t a clear cut evidence where it should be put. Therefore, both opinions are correct. I personally choose to put it on the chest as this evidence is stronger in my opinion.

Yes, you did wrong and you have to pray the prayer again because you have left one of the pillars of prayer and that is to pray standing up while being able to do so. You should have prayed with the other sisters.

You shouldn’t repeat your prayer as long as it was complete and not nullified. You should however offer voluntary prayers to erase any loss of concentration in it.

If it is too difficult for him to walk up and down the stairs, it is permissible for him to pray home but this is something only Allah knows about. Therefore, he should be told that this is between Allah and him.