I think that what you believe you understand is indeed correct.
As far as your questions, honestly I can't answer them but I will comment on them and remember this is from my perspective and there are many other perspectives.
This is a Mauser and will be a hunting rifle, not a target rifle. It is my personal belief that a true 1.5 m.o.a rifle is all that is needed in a hunting rifle. I have taught my children to hunt the same way I hunt, by first enjoying the experience even if no game is taken, and second to have respect for the game and realistic expectations of yourself and your equipment. To that end I don't believe I am capable of ethically taking game past 3-400 yards, although I can shoot farther and hit the target quite often. At 400 yards a true 1.5 m.o.a. rifle will hit within 6 inches of where I point it, and that's good enough for hunting. It won't win any target competitions that I know of though. I have tried to instill in my children the same level of responsibility. If at some point my son decides he needs to be able to take longer shots on game, I will advise him to get a different rifle. Actually I'll advise him to practice stalking, camouflage and patience first and after that if he still thinks he needs to make longer shots to possibly get another rifle to do it with.
The truing of the action that you speak of are, in my opinion, much more appropriate to a target rifle where those modifications can make the differences in tenth's of inches needed. And in this case, I don't have the capability to make them. I also have to keep in mind that this is a sporterized military action. It was never made to the tolerances necessary to make the modifications you ask about worthwhile.
One thing you asked about I will do to an extent, but from the point of view of reliability. The fixture I made in the lathe to face the receiver had another function. I bored it out on center while in the lathe and then turned a concentric shaft to fit perfectly in it. The end of the shaft I have threaded to accept a screw. With that fixture I can screw it into the receiver and use the brass shaft to face the boltface while the bolt is locked to the action. This will (I hope) help make feeding smooth and reliable. I can also use the fixture with a flat disk attached to the shaft to face the inner "C" face I think it is called where the barrel actually tightens against. This will keep the breech of the barrel flat and concentric to the receiver, threads, bolt face.
There are several little things like this I'm doing because I can and they don't take much effort. I'm not going for blueprinted at all.
Someday I think I would like to build a rifle that I do the 'full meal deal' on, but if I do it won't be on a Mauser.
