Illin' Sicilian
5... h6 to chase the knight away weakened your kingside pawns pretty signifigantly, which white exploited later on. 5... Nc6 would have been alot stronger. Threatening to put your knight on d4 would have forced his knight back to f3 eventually
16... d4 was a mistake that should have cost you your knight, luckily white didn't see it.


i dont know the answers to your more general questions.
but thoughts on the specifics:
a) the weakening caused by ...h6 quickly turned into the major source of counterplay for W
b) I would prefer defending the h6 pawn with ...Kh7 rather than g5. 16...d4 was clearly a blunder; instead 16...Ng6. Both these options maintain a more compact, defensible K position.
c) yes, ...Na5 was an error. Your undeveloped Q side needs attention first
