Battery terminals installed incorrectly, now it has issues with the alternator and battery

What are the observations that make it appear it's charging? Where are you seeing that 8 volts? Is that across the battery terminals while the engine is running, or somewhere else?

If it helps, here's a link to an article on how to test the charging system:

https://www.2carpros.com/articles/how-to-check-a-car-alternator

Any time the battery cables or jumper cables are reversed, there are going to multiple blown fuses. Every computer will have diodes built in that are backward and do nothing until the polarity is reversed, then they become a dead short to force the fuses to blow and protect the rest of the circuitry.

The first thing to do is observe if the "Battery" warning light on the dash turns on when the ignition switch is turned to "run". If it does not, the 5-amp "ignition" fuse is blown, (blue arrow).

Next, check the voltages on the wires in the generator's connector. This should be done with it still plugged in, by back-probing next to each wire. A test light will work fine for this. The black wire must have 12 volts all the time. If it's missing, check the 5-amp fuse, (pink arrow). The red wire must have 12 volts when the ignition switch is on. Check the 10-amp "Gauges" fuse if it's missing, (red arrow). Finally, there must be full battery voltage, (12 volts), on the fat black output wire all the time. If that is missing, the really large 140-amp fuse is blown. That one should have blown as soon as the battery cables were put on reversed.

I've been updating my reply after you saw it, so be sure to look at it again. I'm adding diagrams and drawings for you.