My guess is the double bridle was to control the bolt and spin. Horse has decent gaits but is NOT a solid second level, and with those issues not worth 17K unless you were a Pro willing to deal with the issues. Might be the behavioral issues are due to pain - and the less he's worked the less pain - hence why he's not as well muscled as he should be?
Just totally guessing here - glad the gentleman was honest with you about this horse. Keep looking and riding other horses - riding "cows" and such will make you a better rider.