That ground jumper isn't the only player. The fuel line is attached to the chassis with several steel clips. But even if the line and all those clips are rusted bad there should be sufficient accumulative ground path. Then the jumper between firewall and engine block is a player also.
To throw a edumacated guess as to why the fuel gauge needle doesn't quite reach the full mark when there is a full tank?... The inside of the fuel gauge looks about the same as the temp gauge. The crusty crud buildup on the beam/winding restricts beam/needle movement. Whatever the original fault was, it blistered the copper trace off the circuit board. It could have damaged both gauges.
Here's a brain teaser... How did the defective temp gauge pass 2.43 volts? That was shown in a previous pic.
