What are the supplications and remembrances (adhkar) to be recited in such situations, and is it permissible to inform the suitor of one's attachment to the girl so that he withdraws from the engagement?

If a girl's engagement has been approved, it is not permissible to seek its annulment by telling her of your attachment to her, or by supplicating to marry her, as this entails striving to cause discord between them.

There is no need to investigate whether the girl was forced into the engagement or not. It is permissible for you to ask the fiancé to leave her so you can propose to her, and some scholars have mentioned the permissibility of offering compensation for this.

It is advisable to turn away from this matter and be content with Allah's decree, for: "And perhaps you hate a thing and it is good for you; and perhaps you love a thing and it is bad for you. And Allah knows, while you know not." And supplicate to Allah to grant you a righteous wife.