None of the things you mentioned will change the spine except the sandpaper. You can change the spine by lightly sanding an arrow or a fishing rod on one side only. If you sand it all around, the spine stays the same, but sand only one side, and eventually the spine will shift (to flexing more easily on the side you sanded). Believe it or not, some rod builders will sand a blank at specific places to increase the flex in the rod at that point. It was common practice with bamboo, but it can still be done with graphite or fiberglass rods. You just need to be more careful, because there isn' t much material there to begin with.

What you are saying about inconsistencies in spine between arrows makes a lot of sense. The more similar the spines are between arrows, the better the arrows are going to group. I' m not arguing that point, I' m sure you are right that they can be inconsitent. I' m only saying that spine is an inherent quality of an arrow, and that for any given arrow- once it' s made the spine itself will not change unless the arrow either loses a good amount of graphite along it' s length (on only one side), or if the arrow is severely damaged.

I' m sure you have noticed too in your measuring that most arrows (and rods) have more than one spine. Some have 4 or 5 noticable spines. When making fishing rods, you find the absolute weakest spine, and you place the guides so that when you flex the rod to cast, that natural flex axis in the rod will " steer" your lure. But lots of builders feel that this is over-rated, and that really it makes little difference. Case in point- the big rod making companies don' t even bother with spine, they just put the guides on randomly- and you never hear people complaining...