Is the end-of-service bonus from the Employees' Fund, which optionally offers usurious loans to its participants, permissible or forbidden? And does the sin fall only upon those who seek the loans, or upon all participants?

It is not permissible to subscribe to a fund that lends with interest (riba) or invests in impermissible matters, because the subscriber bears a share of the sin due to the fund acting on their behalf. They must dispose of any amount exceeding their principal, unless it resulted from a permissible investment.