What is the ruling on what the questioner did in his youth, uncovering 'awrahs (private parts), and how can he repent from that? Should he inform the girl he wishes to propose to about his fertility problem, and should he propose directly or gauge her interest first?

If the act occurred before puberty, then there is no sin upon you. However, if it was after puberty, then repentance is obligatory, and it is permissible for you to supplicate for the families of the two girls and to treat them kindly. You are not obligated to inform your fiancée about the possibility of infertility, but if doctors definitively state it, then it is preferable to inform her. Do not tell your mother what happened and conceal your matter, and there is no harm in proceeding with the engagement proposal.