((a)) ** .—** The full faith and credit of the United States Government is pledged to the payment of a guarantee made under this chapter, for both principal and interest, including interest (as may be provided for in the guarantee) accruing between the date of default under a guaranteed obligation and the date of payment in full of the guarantee.
((b)) ** .—** A guarantee or commitment to guarantee made under this chapter is conclusive evidence of the eligibility of the obligation for the guarantee. The validity of a guarantee or commitment to guarantee made under this chapter is incontestable.