P4F,
Firstly, thanks for letting me know about the new NRG Systems towers manual. I've been reading it through this morning and found a few gems. I was not previously aware of "snap through" so this alone makes it worthwhile.
Your question about the "positive curve":
So I never really gave it that name, but I was certainly very cautious about not applying too much compression to the column. Before building my tower, I worked out the compression force in the lift process, and compared it to the compression in the upright position (with/without ice) and the lifting condition is pretty severe.
It's not easy to do general theoretical calculations when working with column compression on an assembly like a tower.
One reason for this is that the restraint given by the guy wire stations are "pin joints" and the sleeves are neither fixed nor pinned. They can transfer some bending from section to section of the tower, which combines a bending moment with compression. That's a tricky set of problems. To solve it with mathematics and not make wrong assumptions or over-simplifications took a fair bit of research on my part. I'm still not sure I did it right, but, hey, my tower goes up and down.
The lift cable running from winch to gin to tower is pulling at an angle, so it applies both the lift force and a compression force at the same time. Each of the successive guy stations on the tower that you have also connected to the gin add even more compression if they are tight. So if you had guy wires from the gin pole pulling tightly in the middle of the tower, they would be adding 1000's of pounds of compression to the tower. It would all add up to a very large compression load at the base.
That's my take on why you want a "positive" curve. If the middle of the tower "smiles" a little as it goes up, then that means the guy wires in the middle are holding it up somewhat, but not applying a strong lift load at the middle. They will add much less compression load to the base.
Oh, and I also just realized one more thing. If your gin pole is supporting the tow cable not just at the end but also cables to the middle guy stations on the tower, consider the angles THEY make to the gin pole. A guy wire going to the gin pole from the middle of the tower reaches the gin pole at a steep angle. So it puts a lot of compression on the gin pole, too. Wow, that's bad for the gin pole AND the tower. No wonder NRG warns people about that.
A caveat: My tower has only 3 guy wire stations and the upper 2 are quite close together so I've never been confronted with the problem of middle guy-wires doing to much lifting.
I'm not sure I could follow all that explanation myself without a diagram. Hope it makes sense to you.