2002 Toyota Prius HV Issues

Hello Everyone! I joined PriusChat because I'm having a problem with my 2002 Toyota Prius and my dad and I can't seem to figure out exactly what the problem is.

So about a month ago, while driving, the triangle of death came on along with the check engine light.
We scanned it with a code reader, and got the P3006 - Battery levels are unusually different code.
It continued to run normally, and then a few days later we ran the codes again and it came up with a P3015 - Battery Block #5 weak code. We realized that there was probably a weak cell in the battery so we ordered one on eBay. The car was still running fine, until one time on the way home from the store, the BRAKE dash light came on while driving. As soon as it did this, the car started revving up really high, and the car began to slow down. When i took my foot off of the gas petal, the car accelerated a little on it's own.
The brake was responsive and slowed down the car, and the gas petal would slowly accelerate the car back up, but the engine was screaming.

Once we got home, we turned the car off, started it back up, and the engine was fine.

We stopped running the car, and took the battery out. We discovered that 2 cells were lower than the rest, ordered new ones on eBay, replaced them then put the battery back in the car. We did not balance the cells, however because we were under the assumption that they self-balanced.

After making the battery repair, we ran the car for the first time in 2 weeks and the engine would not start every time, and kept having a cylinder misfire, which we figured was normal since everything had been disconnected for 2 weeks. The codes were P3191 and P0301. After about an hour of acting up, the car ran fine the entire day with no codes and no issues.
The next day, we went to take the car out again, and it ran fine for a while, but then the triangle/check engine light came back on.
P3015 came back (block #5) and that was it.
We cleared the codes, started the car back up and it was fine for a while. After about 15 miles, it lit up again and the BRAKE dash light came on again and did the same thing. We pulled the car over, ran the codes and this time it ran
P3006 - Battery Levels Unusually Different
P3015 - Block #5
P3023 - Block #13

Every 15 miles or so, the BRAKE thing comes on, makes the transmission slip, and you have to start the car back up and its fine for 15 miles.

What is the connection between these two problems, and how do we resolve it?
All help is appreciated!

-Ryan