The fact that Seller was in breach does not necessarily mean Buyer does not have to pay. Buyer is excused only if the breach is material. The contract makes the completion date a promise. Buyer will claim that she told Seller that the date was important. However, I don't see that in the written contract.
Seller can still atone with damages like the cost of moving the wedding reception to a hotel.
Buyer must still pay but can deduct damages. Buyer and Seller entered into a modification. As required by para. 5 of the contract, it was in writing. Even though not required, there was consideration for the modification.