E.T.
Not being able to get financing IS defaulting unless your contract provides a contingency. Think of it this way: the builder is now missing a buyer. It doesn't matter what the reason, he's still stuck with a house that isn't sold. As for the upgrades, that's money the builder spent too-- even if they make the house nicer. The nicest of upgrades don't necessarily make the house easier or the builder to sell since they make the house more expensive.
Sorry to say but I think you're stuck unless your lawyer friend is a real estate expert and wants to write a letter and argue this with the builder for you.