By the sounds of it, each bite you've told us about was either your fault, or the fault of the other party. Most of these events seem like they were surprising for her; most dogs I've met who AREN'T scarred for life, shoot up and try to move when a person steps over them, add fear biting to that and you have the perfect recipe for what's happened. Strangers coming into your house, uninvited or unannounced, call for the same sort of reaction. The first incident four months into your ownership and the big guy coming in unannounced are prime examples of "you're not supposed to be here, go away" behavior.
None of this, to me at least, is worthy of euthanasia. I see a dog doing what she's supposed to be doing to protect her pack in most of these cases, excepting the sunshade incident (your relative of a relative's fault) and her reaction to being stepped over (your barn owner's husband's fault).
Keep her on leash, get her a muzzle. Train the humans around her better. Set up boundaries for the people you're around (don't do this, don't do that, be cautious of...) so that you can all enjoy the company of one another.
Her history and the situations (parties, going to friends' houses, going to the barn, etc) she's been in considered, you could have a lot worse stories to tell and a lot more of them too. Don't give up on her just yet. Do take better measures to protect her from herself and other human stupidity - as well as to protect yourself from the effects of her biting the wrong person.
The path is different for you and me, but the journey begins in the heart.