Rats, Scary Lights, Gas Engine Never Stops...

It's a lot, sorry. I'll start by saying I have no idea what I'm doing, but trying my best. It all started with no tail lights. I replaced the blown #30 tail fuse. It blew again. I started chasing burned bulbs, replaced four. I got to front side marker light and found chewed wires. I couldn't turn the socket to actually see the damage. This car is already missing a piece of windshield washer tubing from previous rat damage. At that point decided I would just take it somewhere to fix the chewed light wiring and washer tubing.
The day I started car to take to shop I had a "check hybrid system" warning, check engine light, yellow triangle and a few more lights. Drove about 30 miles to dealer. Everything sounds normal except gas engine never stopped running. Dealer says code "P0B51". I've got more info. on various wires they checked on certain connectors that "ohmed good". Report says "Inspected wires coming out of hybrid voltage sensor and they were corroded due to water intrusion from leaking tail light assembly." I was unaware of any leak. Report continues: "Need to start with replacing the hybrid battery and hybrid battery voltage sensor. It may also need the junction block connected to the hybrid battery." Estimate total about $5,300. Paid about $370 for this report and estimate.
Drove car 10 miles from dealer and parked it. Cleaned out trunk area, found water standing in spare tire storage area, pulled drain plug, dried out. That storage area was dry a week before it went to dealer. I had a bad experience with another dealer last summer and was giving this one a chance. I'm feeling very skeptical about the whole situation. This is a 2013, bought new, now has 90,000 miles. This was my husband's car. He did most of the maintenance himself and has since passed away. I have yet to determine if the 12v has ever been replaced. Could that cause this code? Any thoughts would be appreciated.

 

I was hoping to keep it for my daughter...

 

Google found a P0B51 Toyota hybrid battery voltage sense "E" circuit low. Lists possible causes as faulty battery smart unit or faulty hv battery assembly. The dealer mentioned corrosion at the battery connections but wouldn't clean them, only replace the battery. I was completely unaware of any water leakage until the day I took it to the dealer. I just don't understand how there was no water in there 2 or 3 days prior and then at least 2 inches of standing water when I got the car back. I've been searching the internet trying to learn a little more about all of this. I just didn't think the car would be driving like it is if the battery needed to be replaced.

We have another 2013 Prius. Last summer I took it to a different dealer for a different warning light and a different chewed wire. They originally wanted me to file an insurance claim and said it would be at least $5,000. After I refused that the wire was fixed, the light went out and the car has been fine ever since. That one cost about $300. I was hoping that I'd get that lucky with this one.