A and C
"Airframe parachutes are not a realistic retrofit" - What rot. There is a kit to modify the Cessna 172/182 with a BRS - probably one of the most ubiqitous training aircraft out there.
BRS Parachutes | Cessna 182 Systems FAQ
Anything can be modified, it just depends on whether you are prepared to pay for the engineering design costs, the flight testing and the certification. I suspect that there aren't enough G-115E owners out there to make the upfront costs worth it. Indeed it would probably be cheaper to bin the G-115s and buy Sportstars or alike...
The B Word