What I was reading said that most of the cells within the battery are still good on a typical "bad" one
Being that one can't service even one, all it takes is one and then the whole thing's NG.
I haven't seen one of those Prius units before but I did once see the battery assembly from a Lexus 400H once and that wasn't totally enclosed like that, you could see all of the individual batteries and connectors YET with that too, it had ONE bad battery and wasn't available separately, sold only as a whole unit for ~$5K warranty cost (so more if CP=no warranty).
You have to be hybrid certified to work on Toyota hybrid systems, lots of voltage, more than enough to kill you if you don't know what you're doing.