Re: How "important" are certain details when reloading?
To get decisive data, IMO you really need to test seating depth in larger intervals. I believe it noteworthy that the Berger seating depth test specifies .040 increments. I have used that test with many other bullets, including Nosler Partitions, with very good results.
You don't say what distance you were shooting. I recommend doing your initial testing at 100 yards. Once you have your final load figured out, confirm it at longer distances.
When you shoot your test groups, hang one target for each group you plan to shoot. Shoot your targets/loads in round robin fashion to more evenly distribute your error (fire one round from group 1 at target one, one round from group 2 at target 2, and so on).
You fired enough rounds that you should clean your rifle before your next set of tests. Make sure to load some extra rounds for fouling shots before your next test.
While testing, it is not important to be dead on in your scope adjustments. There is no point in adjusting your scope until you settle on a final load. Because your POI is likely to change as you go through load development, trying to adjust your scope for a dead on POA/POI will result in you chasing your tail instead of learning something about your rifle/load combination. Use you fouling shots to get in the ballpark and stop there until you have completed load development. Zero with your final load.
You don't need to use virgin brass each time. You should be able to develop a good load using the same cases that you started with.