Yes, the young girl/maiden will attain salvation as she has performed prapatti; prapatti is performed even for infants.
Scriptures will not specifically instruct everything. It does not state one can attain moksham by not adhering to its ordains.
Those who have performed sharanagati must comply with the scriptural ordains. If one follows the ordains for a prappanna and is a paramaikanti following Vaishnava dharma, despite being married to a Shaiva. If one could not adhere to the ordains and is associated with demi-gods, they are given opportunities to redeem themselves if not it shall be left to the divine will of God.
It is better and safer for those who have performed sharanagati not to get married into families of other beliefs rather than pondering later.