Frequency bin of the positive and negative frequency
In the tutorial it is stated that


  Because the frequencies are spaced at 1/72 and the first bin corresponds to 0 frequency, the correct bin is 72/12+1. This is the frequency bin of the positive frequency. You must also include the frequency bin corresponding to the negative frequency: -1 cycle/12 months. With MATLAB indexing, the frequency bin of the negative frequency is 72-72/12+1.


So, regarding your questions:


It's not that there is only one bin for positive frequencies. The thing is that you are looking for a linear regression model, so you check what frequency is the most significant and then you create what would be the resulting vector if a DFT was made to a discrete sinusoid of that frequency only. That is your model.
The bin number is not any weird formula or anything. You have data for every month, but in the tutorial they are looking for cycles per year. You have 6 years. So $72/12 =6$ and that's where it comes from. (As you said, the $+1$ comes from the fact that there is a bin corresponding to the mean value.)