Is it permissible to invest money donated for medical treatment? And is it better to return it to the wealthy donors or to give it away as charity in the event of recovery?

Donors are asked about their intention. If they intended only the costs of treatment, then any surplus or unspent funds must be returned to their owners, unless they forgive or agree to its investment. If they intended solace and assistance without specification, then all the money is yours to dispose of as you wish.