Does the insertion of a small portion of the penis into the wife's anus during the engagement period, knowing the act is forbidden and being ignorant of the expiation, invalidate the marriage contract and necessitate expiation?

If the forbidden act - anal intercourse with one's wife - occurs after the marriage contract (aqd) but before consummation (dukhul), it is a major sin with a severe warning. Its expiation is sincere repentance, seeking forgiveness, and resolving not to return to it. This act does not invalidate the marriage nor cause divorce.

However, if the act occurred during the engagement period, you have committed grave sins: being alone with a non-mahram woman (khalwa), approaching her in an impermissible way, and if intromission occurred, it is fornication (zina) necessitating the prescribed punishment (hadd). Your duty is sincere repentance and to conceal yourself.