Is the marriage considered void, and is it necessary to inform the suitor of the past after lying to him and swearing an oath? And does Allah remove His covering from the wife because of that? And what is the expiation for breaking an oath?

Adultery is one of the most heinous sins, but sincere repentance erases what came before it, and a repentant person is like one who has no sin. You must conceal your past, and you were right to conceal your sin from the suitor. There is no blame in your lie and your oath to conceal yourself, and no expiation is required of you, just as lying is permitted to conceal oneself or others. The suitor's statement that the marriage would be invalid if you lied to him does not affect the validity of the marriage. Allah accepts repentance and conceals the repentant in this world and the Hereafter.