This sounds more and more like a charging issue or a continuity issue. h:
I'd remove your alternator and take it to a place like Auto Zone or Kragen.
Get it tested to see if it is producing the right amount of power.

If the alternator checks out, then I'd confirm continuity with both the power and ground cables. Then I'd check the battery voltage with the motor off and on. If the battery connections aren't solid, then the alternator may not be able to effectively recharge the battery and support the "load" of all the vehicle's electrics.

The SRS system (airbags) and the ABS system (anti-lock brakes) have their own diagnostic computers, separate from the Vehicle ECU. So there should be no correlation between the SRS light and a limp brake pedal.

The brake booster relies on engine vacuum, so if the motor stumbles due to poor spark (again..we are getting back to the electrical system) then that might cause the mushy pedal feel. Alternatively, the master cylinder seals may be aged to the point where they're leaking and unable to produce full line pressure.

Give me a second and I'll see what's up with the voltage regulator... I honestly don't remember whether it's an internal component to the alternator or not. h: