A had a mare who had a nasty spook in her that I could ride through 9 out of 10 times. The 10th time would get me off. I spend time desensitizing her on the ground and while mounted. She was turned out 24/7 with a ration balancer for “grain”. Her primary job was fox hunting but this spook also happened in the arena. I spent years trying to find the trigger to her spook which caused me to start riding defensively and mentally always looking ahead for the trigger. I was even finding excuses not to ride. Thankfully one day a daughter said to me “Mom, you are the most nonsense horse person I know and you have made excuse after excuse for XXX. I wish you would move on from her before you get hurt.” Thank God for family members who speak up.
The mare who replaced the one above is a joy to ride and 100% safe. She brought back the fun in riding to me. A friend took the other mare to be a pasture mate for her horse so it all ended well.
If you enjoy the the challenge of sifting through the unknowns then keep the mare. If you want to get back to “I have XX numbers of riding time available and I want to use it for doing XX” then this isn’t the horse for you based on your written description and not having seen the way you handle the horse on the ground and astride. I applaud all that you have done to find answers and think you have done due diligence for this mare.