I can't answer all, but I'll get things roilling:
2) What is the vocal accomplishment?
Vocal accomplishment would mean singing well.
3) What is the downward spiral talk? Talking about the bad things?
A downward spiral is when things keep getting worse and worse and there's no way to stop.
4) The sentence says; "He wrote at the age of the oldest children present", what does it mean?
Presumably this is from a talk or something to children, of whom the oldest were the same age at which this person was writing. (Mozart?)
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