Hey Dino,
1) There are a couple of different plans for making a spine tester. The one mentioned above is one of the simplest. Just find a bare spot on the wall somewhere and drive two nails in the wall to lay the shaft across. You want the support to be twenty-six inches apart. When you put the shaft on there, find the center and mark where the shaft rests. Next get a two pound weight and hang it from the center of the shaft (thirteen inches from each end). Now, mark where the shaft rests with the weight on it. The distance between those two marks is the deflection.
I'm sure someone will jump all over me if I get this wrong, but I think the formula for the conversion is distance (in this case twenty six inches) divided by the deflection.
Example: If your deflection is a half inch, the spine of that shaft would be 26/.5 = 52 pounds.
Caution: This only works for twenty-six inches. If you use a different length, the weight needs to be different to make it work.
2) I'm not sure what the spine difference would have to be to break a shaft, but I'm guessing it would need to be more than a few pounds.
Shafts are very variable. Even shafts that come from the same tree can vary greatly in spine. Outside diameter has a lot to do with it, but not everything.
3) You can rotate a shaft to match the spine you want. It is probably best to use the strongest side of the shaft, but using the weakest is probably OK too. This is one of the methods that I use to match the spine on my arrows. Find shafts that are close in spine then rotate the shaft to match it better.
Remember, when you mark the shafts the way you want them, the nock goes 90 degrees away from the bend. In other words, using the spine tester I discussed above, the way the shaft flexes is around the riser on the bow. Therefore, the direction the weight pulls the shaft is the way you want it to bend. In order to align the nock correctly, in needs to be 90 degrees away from that so that when you put the arrow on the shelf, the correct side is against the riser.
Marvin