I am a stupid mathmo who doesn't understand Art. More to the point,… - Sally's Journal
I am coming to the conclusion you don't have to understand the plot to enjoy it.
Indeed. Operas in general are not plot driven. The thing is that everything takes a very long time when you sing it (compare the length of your summary with the length of Dido), so lots of plot has to throw out to make room for more arias. Sometimes the composer has a really great idea for a witches' song and so inserts an entirely gratuitous plot element. (It might also be because operatic convention says that love is the highest good, and therefore not something the gods would interfere with, so it must have been witches.)
 Yes, that's an opera thing. When some one falls in love, they fall instantly, madly, uncontrollably in love. They usually die in the next act.