When may oral evidence be introduced after a written contract has been agreed to?

I belive my answers are adequate, but I’d like to hear more. I am working full time and can’t dedicate much more time for researching. Advice and help is what I need!

When may oral evidence be introduced after a written contract has been agreed to?

 

What are UCC gap-filling provisions? Give an example and explain how and why they are applied?

 

With regard to remedies available for breach of contract, when would compensatory and consequential damages apply and when might an injured party seek specific performance? Also discuss mitigation.