Yes the Exacto razor saws can be used for the first two. And when you think about it, no one cuts slots in the saddle right? Well I've seen it done a few times but it's by no means the norm. So at the saddle the string just rides right over the top of the bone piece to the bridge holes. A nut needs a little guidance even if it's pure straight through pull since the break angle is much less then the bridge, but as Charles mentioned it doesn't matter if it's a V or a U shape. The string due to the tension and the downward pull is going to find it's happy spot at the bottom of the valley and all you really need is for it to be deep enough to hold it in place. So if that valley being U or V shaped is a bit too wide for the string [dare I say *gasp] who cares? If anything it's worse having slots that are too narrow and bind up the strings.
The three important things about nut slots is that 1) they hold the string in place and don't bind 2) they are spaced equally so no strings are too close or too far apart relative to the others and 3) the slot depth is equal too or only very slightly taller then the fret in front of it.
You can accomplish that in many ways
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