What is the optimal course of action regarding recurring doubts about estimating the amount of money stolen from the partnership between the father and the aunt, knowing that the father compensated the aunt without her knowledge, and should the aunt be informed of the theft?

The basic principle is that one's financial liability is free from obligations until proven otherwise. If a person is certain that his liability is encumbered with something but is unaware of its exact amount, then he is obligated to exercise caution in estimating it when making a payment. It appears that this occurred at the beginning, and what happened thereafter are merely whispers (waswas), and the liability has been cleared by what has already been paid.