Is it permissible for a student of knowledge, who is dedicated full-time to Da‘wah (Islamic proselytizing) and guidance, to accept a fixed monthly salary from Zakat funds to cover his daily needs, while he has an amount deposited in a company that generates profits for him, but these profits are not sufficient for saving?

It is not permissible to give Zakat to a preacher for him to save it. Rather, it is given to him if he is dedicated to da'wah (preaching) and does not have enough to sustain himself. If the profits from the amount deposited in the company are sufficient for him and lift him out of poverty and destitution, then he is not eligible for Zakat. However, if it is not enough for him, then it is permissible for him to take Zakat to complete his needs, not for saving.