1) Shmini Atzeres is about תשובה, Shavuos is about צדיק.
Therefore the עצרת of סוכות is right after סוכות, as תשובה takes no time, while the עצרת of פסח is seven weeks later as what צדיקים do takes time.
שמחת תורה is joined with שמיני עצרת to add the joy element which breaks all barriers and makes the עבודה of תשובה possible. MP3