Can an investor whose shares have been tainted by usury wait to recover his original capital before liquidation, or must he sell them immediately, even if that leads to a loss?

If a company deals in usury, it is not permissible to buy its shares. If a shareholder is unaware that it deals in usury, then comes to know of it, he must withdraw from it. As for the profits that came to you before you knew the company's situation, you may benefit from them. However, it is not permissible to remain in the company to recover your capital.