Adding 24V Multiplus to Schaudt WA+EBL setup

[autorouter]

yes of course large cables between inverter and bus bar, bus bar and battery. But every guideline and spec I read tells me there should be the same amp capacity of cabling all the way to chassis ground. (which in my case is directly off the starter negative, at the front).

The high power 24V flow is as you say between the inverter pos and bus bar, and bus bar to battery pos. The high power return (0V) is from inverter neg to neg bus bar, and neg busbar to battery neg. That's the complete high power flow circuit. The chassis is not involved in that power flow at all.

However the flow through the XS 12/24 B2B does involve the chassis. The 50A 12V flows from the alternator pos to starter pos terminal to B2B input, then 25A 24V from B2B output to inverter battery pos bus bar. The 25A (0V) return goes from the inverter battery neg busbar to B2B neg, then it's 50A from B2B neg to starter neg, and then to the chassis ground and back to the alternator negative. So the pos and neg cables from B2B to starter battery need to both be OK for 50A.

Once it gets to the starter battery terminals, the pos and neg cables to the alternator are designed to take the starter motor current of several hundred amps, so no worries there.

If you want to avoid running a cable all the way back from the B2B negative to the starter battery negative, you may be able to find a nice solid chassis connection point in a convenient place, to take the return current flow back to the alternator. It doesn't have to go to the starter battery negative, that is only used because it is a convenient chassis grounding point.