11.9 volts is extremely low. Essentially, the battery is dead at that point. At 6 volts it's useless. I think this is the source of your electrical problems. You shouldn't even try to diagnose anything at that low voltage because automotive systems are designed to run at 12.x volts. If the engine will start, then the alternator should provide the necessary voltage of 12.7 - 13.1. If it doesn't then your alternator is not working. Do a voltage check at the battery when the engine is running.

Banging on anything won't solve any problem. It's a possible fix if you're stuck somewhere. For a daily vehicle it's not a fix. If you're not getting the correct pressure in the fuel line then your pump probably needs to be replaced. Other possibilities are clogged fuel line and corrosion. That's my 2¢