Ambiyaa

Q: Someone I know asked me a question regarding Islam, and I didn't have a substantial answer to give due to lack of knowledge. And frankly, I myself am in a delimma regarding it.

In surah al Baqarah 2:30 Allah Ta'ala told the angels that He is going to create a new being who will act as his khalifa on earth. This was prior to creation of Adam (alaihis salaam). As I understand, Adam (alaihis salaam) was predestined to be sent to earth even before he was created. However after his creation, he had to eat from the tree to be expelled from earth.

From my feeble understanding, Adam (alaihis salaam) didn't have any choice because Allah Ta'ala created Adam (alaihis salaam) with the specific purpose of functioning as His khalifa on earth.

So was Allah Ta'ala going to send Adam (alaihis salaam) to the earth regardless of his disobedience? Doesn't this also mean that Adam (alaihis salaam) predestined to disobey Allah Ta'ala?

Q: Is it true that Hazrat Isa (alaihis salaam) was a refugee?

Q: In the time of Rasulullah (Sallallahu Alaihi Wasallam), gold and silver coins were used as currency. So in the time of Hazrat Isa (Alaihis Salaam) and Hazrat Moosa (Alaihis Salaam), were gold and silver coins also used as currency or was something else used?

Q: How Adam (alaihis salaam) and Hawa (alaihas salaam) children got married to each other? How can children marry their own brothers and sisters? 

Q: I have heard that when Hazrat Isa Alayhis Salaam comes to this world he will come as a Nabi. But I am confused because we know and it is mentioned in the Quran Sharif that Nabi Muhammad (Sallallahu Alaihi Wasallam) is the Seal of the Prophets and is the Final Prophet and Messenger, after him no new Nabi or Messenger will come to this World. Please clarify.

Q: Who was Danyaal (alaihis salaam), and was he a prophet?

Q: Is there any authentic Hadith telling that Hazrat Idrees علیہ السلام die on the fourth asmaan (heaven)?

Q: When Hazrat Isa Alayhis Salaam comes to this world, will he come as a New Prophet or Nabi?

Q: Were churches present in the time of Hazrat Isa (alaihis salaam) or they are modern changes? Is it permissible to joke about churches?

Q: Did any of the Prophets ever do any big mistake? Have they ever been warned by Allah Ta'ala sternly?