That car is headed for BIG trouble, if I read your post #4 correctly. If you have one of those HORRIBLE battery cable clamps that you can take apart, you are headed for trouble.

I'd replace it with a cable that is welded (like the OEM cables). With the connection that poor, on those times you did get the car to start, the battery's connection was resistive and therefore your battery's ability to filter the crud and crap from the starter, alternator and any heavy motor in the car is impaired. The spikes that those motors can generate can be substantially higher than 14 volts, putting various modules at risk.

I have been involved in automotive accessory electronics since 1983. But it's your car, so do as you please. Just sayin.