How can one atone for sins and return to God after committing serious transgressions during the engagement period, and what is the ruling of Islamic law concerning these transgressions? And is it obligatory to disclose what happened to the prospective husband?

Praise Allah for your repentance from sins. Repentance wipes away sins, and one who repents from a sin is like one who has no sin. Strive in obedience to Allah and increase in good deeds that expiate sins. Among the signs of accepted repentance is that you become better than you were. Erase from your memory what happened to you, and there is no need for excessive self-reproach. Since penetration did not occur, what happened to you is not zina (fornication/adultery). Do not tell anyone what happened to you. Cover yourself with Allah's veil, make your repentance between you and Allah, and hasten to marry a man of good character and religious commitment. Do not tell him anything of what happened to you.