Is an engagement considered a marriage if it fulfills the the conditions and pillars of marriage, such as publicity, declaration, witnesses, and the guardian's consent, or not, and why?

A marriage contract might be called a 'khitbah' (engagement) in some customs, but what matters is the conclusion of the contract with its legally recognized conditions and pillars, such as offer and acceptance, witness testimony, and the absence of impediments. If these are fulfilled, then a valid legal contract has occurred, even if it is called a 'khitbah', because in contracts, the meaning takes precedence over the wording. As for the 'khitbah' that resembles a contract in some ceremonies, it is merely a promise of marriage and can be revoked. Marriage is concluded by whatever people consider a marriage, in any language or action, and by the indications of the circumstances. If the gathering was merely for engagement, then it is not a marriage contract. But if it was for the purpose of concluding and finalizing the contract, then it is a marriage contract, even if it is called a 'khitbah'.