Is Istikhara mistaken if it led me to an unlawful act whose unlawfulness I was unaware of until a period of time had passed, knowing that I donated my entire salary from it? And must I dispose of everything I received from this salary, even though I did not personally benefit from it and donated it? And must I do anything else for repentance and remorse?

Your Istikhara (definition) is not invalid. However, Istikhara alone is not enough. You must take all necessary measures, investigate thoroughly, consult with people of sound judgment and experience, and learn from this lesson by asking and scrutinizing before undertaking any task. If you have fallen into something forbidden, then repentance and remorse are sufficient. Istikhara is only prescribed for permissible matters; it is not for an obligatory, recommended, forbidden, or disliked act. The nature of the work was not clear enough to determine the ruling on the money received.