Killing robbers

Q:

1- As in this time robbery is common in our society. Some time robbers snatch costly things on gun point. I want to ask what is Islamic teaching about protecting our goods. Does Islam teaches us to resist in this situation from snatching or protecting his/her life is more important? As if one resists before robbers they may harm that person or even kill him.

2- If during robbery one kills robbers is this allowed or not? Will this person be treated a killer in the sight of Islam or during defence this is allowed?

Giving gold as mahr

Q: As per the need of the time of those days, the devaluation of money is going so fast and islaam made the nisaab of zakat in gold and silver. According to this if we make the transactions of mahr in Nikaah in gold, will it be valid shariah or not. In the case of divorce I find a man divorced his wife after 10 year's of the nikah, that time the Mehar was Rs. 1125/= that's also not paid at the time of nikah. With the papers of divorce, he sent the same amount to her after 10 years. So if they made the transaction in gold, the divorced woman would have something to support her for the future.

Not joining the jamaat Salaah

Q: This weekend I was out musafir for a few days with a few friends. We came to a masjid to perform salaah. Two friends we finished making wuduh and the third was still making wuduh so the two of us decided to start our farad thinking the 3rd brother will join us. Our first rakat was almost finished when the third brother finished his wudu. The brother didn't join us but performed his salah on his own. When asked why he didn't join us he said because we didn't wait for him. Is this brother sinful for doing that?