Hi there. I would recommend to work with 1/8 of the mesh. 1. Start with your basic shapes 2. Create seperate mesh that would act as boolean 3. Boolean 4. Rotate the mesh 45 degrees 5. delete 1/8 of the mesh 6. Clean up 7. Rotate back -45 degrees 8. Symmetri and voila. HERE IS THE 1/8th of the mesh cleaned
I know triplanar is XYZ-axis projected which is basically using vertex normal I guess, but I cant really its realtionship between 45 degrees angle because it's more visible at 45 angle degrees?
45 for for skinning is pretty optimal. If 0 is arms at the side and 90 is T-Pose then 45 is right in the middle. The shoulders need all the help they can get. It makes modeling a bit wonky so I normally suggest model in T, and before unwrapping pose them at 45.