18a The wave that engulfed the house was a real misfortune (7) S???R?E
12d Forgive and, with a change of heart, sign (5) V?R??
24d On the lake, one will tie up (4) ?A?E
the last letter of 12d is also the third letter of 18a
the only other word that will fit is SPLURGE, but I like SCOURGE and I think the explanation you gave for 12d is apt. I tend to agree that is VIRGO (sign)