The validity of the contract shall not be contested except for nonpayment of premiums, after it has been in force for 2 years from its date of issue. No statement made by any person covered under the contract relating to insurability shall be used in contesting the validity of the insurance with respect to which such statement was made after such insurance has been in force prior to the contest for a period of 2 years during such person’s lifetime, and not unless it is contained in a written instrument signed by the person making such statement. No such provision, however, shall preclude the assertion, at any time, of defenses based upon the person’s ineligibility for coverage under the plan or upon other provisions in the plan, except for any provisions establishing, as a requirement of eligibility, the furnishing of satisfactory evidence of insurability to the health maintenance organization.