Does marrying the girl with whom I was in seclusion before becoming religious alleviate the sin of being in seclusion with her?

Allah expiates sin through sincere repentance, which requires abandoning the sin, regretting having committed it, and resolving not to return to it. If the woman has repented sincerely, it is permissible for you to marry her, and you will be rewarded for that, for shielding and protecting her. If you do not marry her, there is no sin upon you, and that does not affect her repentance.