Is the aforementioned contract valid in its form, is the word "qabiltu" (I accept) sufficient for acceptance, and what is the sunnah form for a marriage contract?

The marriage formula is one of its pillars, and it includes the offer and acceptance from both parties. The proxy acts on behalf of the principal in both. If the suitor says to the guardian, "Did you give her in marriage?" and he says, "Yes," and then says to the husband, "Did you accept?" and he says, "Yes," then the marriage is concluded in the presence of two witnesses. There is no specific formula required for marriage; whatever people consider a marriage, in any language, word, or action, it is a marriage.