What is the ruling on working in a company most of whose income is derived from bank interest? Is the salary from it permissible or impermissible? And what is the ruling on seeking the best bank interest to invest the company's money?

It is not permissible to work with usurious deposit accounts, nor to seek out the most profitable bank and deal with it for depositing funds, even if it is for the company, due to the prohibition of aiding in usury. However, if there are fields of work within the company that are unrelated to usurious transactions, then there is no harm in working therein and benefiting from the wage.