Is the phrase "Congratulations to you both, she is yours" considered a sufficient formula for a valid Islamic marriage contract, such that the suitor is entitled to treat the fiancée as his wife after her father's death?

Marriage is not valid except with the صيغة (formula) of marriage (التزويج or الإنكاح) according to many scholars. The formula mentioned by the questioner between him and the woman's guardian does not constitute a valid marriage contract. The contract must be completed with an offer from the guardian and an acceptance from the husband, in the presence of two witnesses.