Ustadh, what should one do if they had to say “wallahi” over a situation that they had no choice in? Is there a kafarah for it ? Or should one just repent to Allah for it ?

If the oath was on a future matter and it was broken, the kaffarah of an oath is due. If it was on a past matter, then only repentance is due. The one compelled to swear to repel harm from himself, there is nothing upon him.