#1
I am currently building a guitar neck and was wondering if you needed a nut at the top. (i bet you do) but just wondering.

Rock on!
#2
Actually you could end with a fret and then some guides to keep the strings lined up - but a nut of some sort is trad.
#6
The nut is required to keep the strings correctly spaced when you do things like bending them. There is no way around it, it is required. Some guitars use a 'zero fret', which is just another fret in what would have been the position of the nut. The nut is moved farther towards the tuners to compensate for this fret in its original position. The purpose of it is to make open strings sound more like fretted strings.
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