What is the ruling on one who swore not to expiate, then expiated and apologized? And is expiation for an oath a condition for repentance for one who swore to abandon a sin then committed it?

Expiation for an oath is not a condition for repentance. Repentance is an independent act achieved by abstaining from the sin, resolving not to return to it, and regretting having committed it. The expiation remains the responsibility of the person after repentance, and they are required to perform it. It is more prudent to hasten in offering the expiation to clear one's conscience and to avoid disagreement.