Generally no appeals. FEMA only hears appeals of cost adjustments made after you accept the offer, such as for insurance, noncompliance or audits, and not appeals of denied extensions to accept an offer. The obtain-and-maintain insurance requirement still applies, and the DHS Inspector General can audit any alternative procedures project.
Proof that you obtained and maintain insurance on facilities funded through alternative procedures, and full procurement and cost records, since the OIG can audit these projects.
Accepting the fixed cost offer effectively ends your ability to dispute damage, scope or cost. Treat the acceptance decision like a settlement and have your engineer and cost estimator sign off first.