bsterns, on Mauser actions the rear action screw particularly can be TOO tight, which will tend to put some stress on the action. That's why the advice about firm rather than gorilla tight.
The 1-12 twist should handle any bullet up to 200 grains.
As far as the thin barrel being better with 125's, I have no idea what that's all about.... my thin barrel 308 shoots 165's very nicely
Reloder 15 should be a good choice. I like Reloder 19 powder quite a bit, but not in the 308- it's a bit too slow for the 308, IMO.
You might try a different bullet with your Varget, too.
Good luck, and keep us posted.