[cerelestecerele]

Do you only have the one bareshaft? Especially with lower tolerance carbon arrows I wouldn't trust a single shaft to tell you more than what that arrow does, Murphy's law says you always pick the weird one as a bareshaft. 3 is enough to be confident, and refletching takes just a few minutes after.
1. As long as you have no chance of pulling the arrow back until the point falls off the rest, it's safe. People used to use much shorter arrows back when it was only heavy aluminiums.
2. Before cutting anything, you need to figure out if the the bareshaft is reliable and if you're getting a false tune or not (if the arrow is hitting something on the way past). You can confirm that but changing your draw weight, point weight etc for a few ends just to check the bareshafts move in the correct direction. An inch generally gives you about an extra 5#. But who knows how much you need.
3. They run stiff for more people than they run weak, but it varies as there's a considerable human and equipment factor. Same way a size 9 foot and a size 9 shoe may be a great or terrible fit depending on the brand and shape.
4. You answered yourself.
5. First time with a new shaft type, I cut only two in half inch increments and track how they fly after each cut first as bareshafts (to check they're the same) then fletch one and check the difference. That way, if they're an unusable size, I still have 10 intact shafts to get rid of.