What is the ruling on marrying a non-virgin Filipino Christian woman who has a daughter from a previous marriage, and is it permissible to marry her with the intention of emigration and completing studies abroad, knowing that the family is unaware, and is Istikhara sufficient in this case?

You must repent and leave the foreign woman immediately. It is not permissible to marry a Christian woman if she is unchaste. Even if she is chaste, scholars have disliked marrying her due to শরعي (Shari'ah-compliant) reservations.

There is no room for Istikhara (guidance prayer) in a forbidden marriage. If the marriage is permissible, then there is no objection to performing it.

You should seek a righteous Muslim woman.