Gen II Prius Airbag Light Remains Illuminated

Hi all. It's been years since I visited Priuschat but now my car's developed some pretty major faults so I need some wisdom...

During the past two to three years of driving I've noticed that the airbag light stays illuminated throughout the power-on/driving session for several consecutive months. This is bad right, however the SRS system periodically decides to extinguish the light after its initial power-on diagnostic check like its supposed to, and it continues in this course for several consecutive months, as well. At first we thought the light-on cycle followed the onset of winter chill and remind on until the arrival of warmer weather but this last 12 months seems to defy that notion; the light stayed on starting mid-summer 2018 and has remained lit ever since.

Several years ago I remember reading here about under-seat wiring harness manipulation as a possible fix to this issue and I tried all the simple 'fixes' that were recommended but to no avail. This brings us to the second week of February, 2019.

RED TRIANGLE OF DEATH!
Oh my! The dashboard recently lit up like a Fourth of July celebration. With 164,000 miles on the odometer I suspected either the 12VDC or 201VDC batteries. I load tested the 12V battery and sure enough, it scurried off with its tail between its legs. I replaced that with a new Toyota AGM unit but it did not solve the RToD, etc.

I decided it was time to visit the dealer - Mark Miller Toyota, Salt Lake City, Utah - and after FIVE hours of patiently reading H.G. Wells' "War of the Worlds" was told that DTC indicated HV battery replacement to kill the RToD and removal and replacement of the passenger seat airbag assembly due to 'an open circuit.'

I'm doing the work to remove and replace the HV battery modules with a new set tomorrow -- I'll be using 1,000V certified gloves until it's safely removed and disassembled... -- so that will hopefully resolve the Triangle, etc.

AIRBAG
In regard to the airbag issue, I don't see much on this site addressing airbag DIY removal and replacement. I assume this may be due to the squib hazard involved. Is this true? Should I pay the extra $400+ beyond the price of the part and have the dealership service folks handle this one?

Thanks

By the way, the vehicle has never been in an accident.