MAD ammeter bypass question

Sounds like you may be missing the key point to the demo/videos. This is not about charging current from the alternator to the battery while in operation. It’s about total circuit protection from a short or overload condition. It really doesn’t matter what type of circuit protection devices are used (yes,12ga fusible link is present on that shunt wire). Some promote large battery cables and no circuit protection device at all on the shunt wire. The issue is if/when shorts take place anywhere is the stock unfused wiring components, or the shunt wire for that matter, now there is two paths for current to flow from the point of the short to the battery including the stock charge/load distribution path. Segments of the stock unfused wiring are now exposed to a large amount of current (much more than 12ga wires can handle safely) before any circuit protection can react, 120 amps in this case. You can’t wire circuit protection devices in a parallel circuit that will have any conductors that can’t handle the combined total current allowed by each circuit protection device and call it safe.