Is the return from freezing part of one's salary to invest it in a company owned by the person and his brothers, with the intention of receiving a return equal to that of the bank, considered prohibited usury (riba)? And is zakat obligatory upon him, knowing that the company pays zakat on its funds that have completed a hawl (lunar year)?

If the returns from the frozen amount in your private account are a result of the bank investing your money, and not a fixed percentage of the capital, and the bank is non-usurious, then there is no objection to it. Zakat (definition) is obligatory on this amount if it reaches the nisab (definition) by itself or by being combined with your other wealth. However, if the returns are from usury (riba), then zakat is not obligatory on them; rather, they must be disposed of after repenting to Allah.