I'm only just new to this forum, but I've been reading every post on this thread and it's really been a tragic drama. I'm so sorry this has been such a traumatic experience for you. X
Anyway, the older (and wiser *wink*) I've become, I've come to realise that weddings really are a complete load of s**t. It's great business for you cake makers, I understand. But seriously, at the end of the day, it's ultimately about one committing themselves to another one. Now, I'm no math expert but I do know that one plus one equals two. Not 100+ guests and a $20k+ deficit.
If a cake is the straw that broke the camels back, then I worry for the couple.
If you can, smooth things over with them, you will feel better in your heart. Let them buy another cake. Go to their wedding. Have a great time. Make peace and move on. It is just a small moment in time and in a few months it will all be distant past.