1. Yes, it's fine, and yes, it's possible to say this without the inversion. 2. Yes, this one is a good sentence. 3. It has to be 'so' because of the adjective 'excited'. The inversion is fine, but in fact this sentence would sound better the normal way round. Or you could maybe rephrase the last part as 'they could hardly sit still' - this would balance the sentence better. And there's not a lot of point, really..... We don't even do much of this kind of inversion in everyday language.

Pretty good! For #3, It's just "So excited", "such"wouldn't be interchangeable in this case. To use "such", the sentence would need to be rephrased as "There was such excitement that they couldn't sit still." 1. She decided to leave early, as did I. (can't we say 'as I did'?) 2. Gone are the days when I could have been happy. 3. *So* excited were they that they couldn't sit still.