Low mpg/power, hybrid battery gets low, enginge shuts off (part2)

Hi all, it's me again!

2008 Prius 128k miles, bought in California, now in Massachusetts

The car has trouble accelerating, MPG is low, and drives longer than 20 minutes, the hybrid battery (on the display) drops to 1 bar, throws the red ! in a triangle, displays a "hybrid system malfunction" and if I keep driving it dies.
In the past 3 weeks, I have dropped it at the dealership 3 times to get it fixed.
I pick the car up, within 30 minutes to an hour, I have the same problem every time.

First time
They diagnosis it as the aux battery. Which I had replaced at a private mechanic with the Optimum after market only 3 months prior. They replace it for $500.

Second time
They say they need to replace the Mass Air Flow & O2 sensor. $700

Third time
They say it's the catalytic converter. Going to be $2,500.


They claim there was no way to know about any of the above ahead of time and that each thing is simply breaking on its own and that I have unfortunate luck.

Is this correct?
Will repairing the catalytic converter fix the problems I am experiencing?


I've been meaning to sell this car for about a month now & it just hasn't been possible when it's constantly got a check engine light and I don't want to **** someone.

 

I see, that is very understandable. My understanding is all of their diagnoses thus far have been from the codes. Every call begins with the codes being thrown were for x,y,z. I'm only a young lad and perhaps too gullible, but I assumed they wouldn't lie about such things.

Am I being too optimistic?

edit:
I believe they have honestly fixed things that were in disrepair and throwing codes (like the aux battery/mass air flow/o2 sensor). But I don't think they are the cause of the root problems I've been experiencing and I don't think the techs are being too savvy about figuring it out.

 

Any recommendations on how to find one that doesn't suck?
I'm in Boston. Currently using Herb Chambers, which seems to own almost every dealership.