Is it permissible to complete the pillars of a marriage contract with a newly reverted Muslim foreign woman by dividing the witnesses into stages, such that some relatives bear witness first, and then friends bear witness later after descending to the city, taking into account the difficulty of gathering them in one sitting, all of this being before consummating the marriage with her?

A valid marriage has conditions, the most important of which are the guardian (wali) and witnesses. The marriage is not valid without the permission of the guardian and the presence of witnesses. If the girl is a chaste woman of the People of the Book, you may marry her, and her guardian would be from the people of her religion. He can delegate someone to act on his behalf, and delegation by phone is permissible. The witnesses to the marriage contract must be males, and the preponderant opinion is that they must be Muslims, even if the wife is from the People of the Book. If you cannot find Muslim witnesses at the place of the contract, you can postpone the witnessing until before consummation, in accordance with the Maliki school of thought. Abu Hanifa held the view that a Muslim's marriage to a woman from the People of the Book is valid with the testimony of two people from the People of the Book, and this opinion may be adopted when necessary to avoid hardship. You must treat this girl as a non-mahram (definition) until the legal marriage contract is concluded, and if marriage is not feasible, then sever all ties with her.