1. S should bid 1
♠ in response to the double. Bidding 1
♥ might be as good as 1
♠ should N pass, but when N takes another call, and this is entirely foreseeable given the start to the auction, S is poorly placed to find a 4-4 spade fit without forcing the auction too high should it not exist.
Compare the actual auction: S bids 1
♠ and now, if he wants to bid (see later), he bids 2
♥, which would allow N, with 3=2 or 3=3 majors to give preference at the 2 level. As it was, for N to give preference, he would have had to bid at the 3-level.
2. S's second call was the more costly error. He should have got away with the 1
♥ call by passing 1N. I assume 1N was taken to be about 15-17. S has a clear pass of a strong 1N opening bid, so has an equally clear pass of the 1N rebid
3. I have no idea what N thought he was doing with 3
♦. Admittedly, he should take S to be at least 4=5 majors, but S has to have a poor hand not to have bid more strongly earlier. I would bid 3
♥ as N. Of course, that works out even worse than the actual auction, but at least no one can accuse me of resulting here
4. S had become the victim of his own misbidding, compounded by N's 3
♦, so 3N was the least of evils.
'one of the great markers of the advance of human kindness is the howls you will hear from the Men of God' Johann Hari