questions about battery disconnect wiring

Lots of good information here and I agree on many of the statements above.

All of my additional circuits are fused, have properly sized conductors and have solder connections throughout.

I actually manage a facility that designs and manufactures power distribution boxes that are used in the semi conductor and robotics/automation industries. Our products must be built to some fairly strict UL requirements before being marked with a listing and being released to the market. I have been able to leverage a lot of this knowledge on my own projects.