Please help!!! Intermittent Brake, VSC, ABS (!) lights

Hello!! I have a 2004 Prius that just hit 130,000 miles. For the past two months or so now, I have had the problem where my prius brake light, VSC, and ABS (!) lights have been intermittently turning on and off. I bought the car back in September from a friend, who had the inverter coolant pump replaced right before he gave the car to me. The car drove perfectly up until the 12v battery failed. I got the battery changed back in December, and that's when the problem with the intermittent brake lights began. (Which, might I add, I got ripped off for -- the guy charged me $400 for labor and parts)

As soon as this problem started, I brought it into my mechanic who said the brake fluid levels, the battery and everything else was fine, so he advised me to keep driving the car under the assumption that it was a problem with the sensor. I drove from LA to San francisco with no problems, but a couple weeks later the same thing started happening again. Even when the lights would turn on, there would be no noticeable difference with the brakes while driving.

One night after driving for about 15 minutes, the main red triangle warning light was triggered. I parked the car planning to bring it in to the shop the next day, but the morning after the 12v battery was completely drained. I towed it into a different mechanic, who scanned the codes and pulled out a P0A08/264 (DC/DC Converter status circuit). They basically recharged the vehicle, test drove the vehicle and cleared the codes. They said that I kept driving the car normally and if the symptoms returned they suggested that I get the inverter assembly replaced.

So I kept driving the car, and as I expected, the brake lights started turning on again. I brought it back to the same place, and they pulled the same P0A08 code but this time it had nothing to do with the previous code, but with the HV battery pack. I told my mechanic in LA, who drove up to San Francisco for me to replace the original 2004 HV battery with a reconditioned battery from a 2011 Prius. I really thought that would be the end of it.

The car drove fine for the next day or so, but the same thing started happening again!!! (but no red triangle, just the brake lights) So now I'm at a loss. I'm going to buy my own scanner to see if the codes have changed, but I'm not really sure where to go from here. Could it be the same problem before with the inverter assembly? Or could it be reconditioned hybrid battery failing?

I'm a female college student with very little finances, and replacing the HV battery was already so expensive so I'm trying to save as much money as I can. I don't know much about cars and I'm afraid of getting ripped off again, so any help would be GREATLY appreciated. Thank you all for taking the time to read this!!!

Best,
Michelle

(another thing: when my mechanic from LA scanned for codes, he said that I had to replace the speed sensor on the front axle soon, but that it wasn't urgent and it didn't have to be replaced immediately. Could this have triggered any of the intermittent symptoms from before?)