Prayer

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.

If the reason was legitimate and beyond one’s control and you couldn’t by any chance pray it on time, it is permissible to join it with Isha

You shouldn’t cross between his feet and the place where he puts his forehead.

This is not true unless he deliberately skips Asr prayer until Maghrib is due every single day. Whoever does this would lose all his good deeds.