>> what do you do when a donor defaults on a pledge? What do you do when a donor sues you because they're unhappy with how you spent their money?

Talk to a lawyer. Sometimes pledges are just empty promises. Sometimes they are actually contracts. The former can be ignored. The later must be handled by legal professionals.

Examples:

I promise to give you $100 = nothing. I promise to match donations up to $100 = contract. I promise to leave you $100 in my will = nothing. Let my kid hang a painting in your gallery and get $100 in my will = contract.

The use of famous names can be tricky irrespective of whether they once donated or not. "The Micheal Jackson school of dance" ... expect lawyers. "The Trump memorial outhouse" = free speech ... also expect lawyers but feel good that you are in the right.