a.mike.a

Please help...I am a new member, and I hope someone can steer me in the right direction. My stepson has a 2007 v6 that was a one owner vehicle, my sister in law sold it to us.
Approximately 4 months ago the starter seemed to give out after a few revs on the flywheel, as if power was being shut off, and I could hear the bendix (do they still call it that?) release and wind down. His uncle installed a new starter, but after what seemed like only a couple of weeks, the problem worsened, and my stepson drove the problem into the ground.
When I got to it, it would turn maybe a half rotation, then it sounded like the entire crank was going to fall out of the block. I figured the starter wasn't secured properly, and when I looked at it, I was able to turn solenoid 1/8 inch back and forth. I removed the starter to exchange at the parts house, and there were only about two teeth remaining on the shaft.
I looked at the flywheel as best I could, and there didn't seem to be any visual damage, but I didn't spin the crank to inspect it either. I installed a new starter, as well as a new battery, the old battery had been drained and damaged; it wouldn't take a charge.
I put everything together, lowered the jack, hooked up the new battery, turned on the ignition, and it turned over maybe once, and this time it sounded like someone was hitting the motor with a sledgehammer, but it started and idled decently, but it acted as if the battery were on its last leg.
I realize this may sound long winded, but I looked into what it would take if I were to tackle a new flywheel installation, and I would not have the time, as my son is using his mother's vehicle, she mine, and I have my bike, and it's only a matter of time before he breaks something on her vehicle; I'm just sayin...
If I do put it in the shop, $$, I want to make sure I am putting it there for the specific task intended. I don't trust many people to work on my vehicles.
My questions: What caused the initial problem? Is this limited to a certain range of VINs? Why would the gears be annihilated in the new starter?

Any help or advice you guys could shed would be greatly appreciated.

Many thanks,

Mike