Bill,

 

From your description, your ground architecture sounds very similar to mine.  I have the forest of tabs bolted to the firewall with a brass bolt.  On the cabin side, the battery ground is connected to this bolt.  Everything else grounds to the forest of tabs.  On the engine side of the firewall, the engine ground is connected from the front (in the car) cover to that brass bolt.  My arrangement has not had any electrical problems that I am aware of.

 

I also have a stainless wire braid covered fuel line from the firewall bulkhead fitting to the fuel rail on the engine.  I have seen no damage to it.  The fact that your wire braid on the fuel line has been burnt suggests that it might have intermittently served as the engine ground at one time or another.  The braid must not have conducted a large current for very long or it would been destroyed completely.  I can't think of a worse place to have an uncontrolled resistance heater than on that fuel line.  Verifying the integrity of the ground from the engine to the airframe and from the battery to the airframe certainly seems like a worthwhile endeavor.

 

Depending on the details of the alternator installation, an intermittent connection inside the battery also might be a possibility although this would not explain the damage to the fuel line braid.

 

FWIW