I have had a one person thoroughbred mare. She would behave for others though, and would let other people ride her, though she would give a bit of trouble if they were feeling insecure and giving her mixed signals. I never had to worry about her being handleable in an emergency situation, but she definitely didn't get that extra spark with others that she got with me. And yes I agree with being a nervous wreck when someone else would ride her lol. My Arab is another story entirely. She also came from a neglectful home where one of the mexican workers decided that it was a good idea to send 3 or 4 people into her stall to corner her, and then rope her down to get her feet done. She and I have definitely bonded, and she is doing really well for me, but I feel for anyone else who has to go in and try to catch her or mess with her. She has only let one other person go in and catch her with any sort of confidence, but with all others she's a nervous wreck. At this current point, since I live on the property, I am just working on getting her to the point where others can come up and say hi while I'm holding her, and she's getting better. It is worrisome though if I ever can't get there or am not there in a reasonable amount of time, in case of an emergency, cause she can't be caught.
Take for instance Wednesday morning. She decided to be brilliant and jump over the rusty metal bucket cart, and caught her hind leg. Fortunately I was right there, but even for me it took a bit to catch her, as she's never actually hurt herself before, and was terrified and unsure of what to do. I could tell that she didn't want to leave mom, but at the same time, she hurt and didn't want someone approaching her either. Once I caught her, and ended up getting the vet there, we found out that she severed a vein, and had a nice laceration that needed to be stiched up. I cringe to think of what would have happened if I hadn't been there, and no one was able to catch her.
It is nice that she does what I want, and I am nervous once she's trained undersaddle to let someone ride her, as I have very specific ideas of what and how I want things done with her. But I do want her to get to a place where others can handle her easily, and that she respects them. I commend you for wanting to help your horse and yourself, and I do believe that you have done the best that you can with what you have. Because my horse is so terrified of others, especially if there are more than just a couple people around, it makes it hard to get her calm enough that someone else can approach her, and I don't want to endanger her, myself, or anyone else just in the name of trying to make her accept others in a leadership role. Definitely have people over, just to hang around, and be a presence. They don't even have to try and work with her, just be around her where she feels the most comfortable, until she is okay with others being in the same area, then work from there.