Are you representing seller or buyer? Just curious.
If you checked 6 (b), and picked 30 days for 6 (b) (i), then the seller can only accept backup offer (yes, written and accepted). Also, the seller can not invoke 6 (a) until 30 days has passed. On top of that you have the 72 hours clause after buyers can not meet the 6 (a) condition
More details will help me to help you. .