What should someone do who has been forced to deposit their money into an account that accrues unlawful interest? Is it permissible for them to take the principal and donate the interest?

The money taken after five years consists of capital and its usurious profits (interest). Since usurious money is unlawful, you are not permitted to take anything in excess of the principal, and you must leave these usurious profits to the bank, based on the Almighty's saying: "And if you repent, you may have your principal sums." If the bank insists that you take these forbidden profits, then you must dispose of them in various charitable ways, and it is not permissible to benefit from them.