You placed your bet wisely. 'So happy was she...' is a correct inversion. That's the good news. Now, the bad news is that it's a completely unnatural sentence. No English speaker would ever say this. Inversions of this type are extremely rare in modern English, and mainly used in archaic and formal written texts. We would not use an inversion in this way in an everyday sentence about a person being happy and deciding to celebrate. Now it's my turn to make a bet - would I be right in thinking that you have a very old-fashioned grammar textbook?