Why doesn't the Eldorado start?

I am a novice d.i.y.er. I have a 92 eldorado 4.9 base with 125,000 mile on it. The
battery, alternator, water pump, serpentine belt and tension pulley are all new. At first
the alternator would cut out while driving and my battery would quickly drain within a
few miles, sometimes requiring a jump. Now the car will not start. I tightened all
connections and cleaned the grounds. The ground near the starter is bolted into the
block. That ground bolt just spins in the block grooves. I purchased a MM and tried to
do a voltage drop test to the starter but it was difficult to reach and I have no help near
me. However, while attempting to establish a volt reading at the battery terminal on the
starter I accidentally jumped it and heard the starter motor turn over while I was
showered in sparks. Now when I try to start the car all I get is a single, weak click
sound from the engine compartment and a click or two from the ecm area on the
passenger side, or possible from the start relay near the drivers door, it's difficult to
pinpoint. The battery shows 12.6 V and the alt was producing 14.5 V when it did supply
charge before cutting out. If anyone could explain these symptoms I would be
immensely helped by your insight.