What is the ruling on a marriage contract with a girl who was fornicated with before marriage, if there was no repentance except after the marriage? And is it permissible to inquire about the extent of the girl's guardian's (her paternal uncle's) commitment to prayer, knowing that the girl and her mother are Muslim?

We congratulate you both on repenting from the abomination of adultery, for which Allah has promised a doubled punishment, except for those who repent, believe, and do righteous deeds. Indeed, Allah is Most Forgiving to those who repent, believe, and do righteous deeds.

There is no impediment to marrying this woman after her repentance and purification. There is no need to investigate the true identity of her guardian (wali) after confirming his Islam. If you are blessed with a child, he will be lawfully attributed to you, even if the marriage occurred before repentance, due to the difference of opinion among scholars regarding the validity of such a marriage.