Advice on autocorrelation via FFT
Didn't check the code, but the steps you describe in 1. seems correct.
The units of the autocorrelation are time and not frequency, and it is centred around $\tau = 0$. That means that in your $-1024 \leq\tau&lt; 1023$. So you are looking at the wrong peaks.
Now as you can see the autocorrelation doesn't produce a sharp peak which means that it is not a good measure for your case. In general, the autocorrelation doesn't work well with narrow band signals.  

I would suggest you try either the MPM or the YIN methods which are based on the autocorrelation, are extremely simple to implement and from my experience yield goo results.