You have the right idea but I caution you about taking this and that and what fits into your training to teach your young mare to drive. Do it right the first time, read up on how to properly train a cart horse, my words of wisdom to you is GROUND WORK must be taught and taught till the horse can literally do it in her sleep. She should know that when you say the word "Whoa" you mean whoa and it is to be done now. If you have an arena to teach her and to use, please, use it. Driving a long a road with a green driving horse is an accident waiting to happen, people are stupid when it comes to horses, they honk, they gawk, they speed up on you, all sorts of things that a green horse is not equip to handle and if your green too, neither are you. I say this only out of experience and from having endured a bad wreck.
As for driving bits, I have driven in a snaffle, and don't let people tell you that it is for riding, poppycock! They are a driving bit as well. And just remember, any bit can be harsh, it is the hands that are at the end that will make it soft or harsh. I went from a buxton bit, which is for a wheel team in a larger hitch, so I changed them for a military snaffle. When your in a crowd or are doing something that you need a good "stop" on the horse, you want a bit that you can, again if necessary, you can drop your lines down a notch to get your point across or give you a little bit more insurance that your animal will stop or will listen when it is nervous. I have had to drop my lines down a time or two when my team was nervous, but, when you have two 1800lb mares at the end of those lines and your in a large show, I would rather have their attention on me than the thought of them spooking and bolting. At the end of the day, they are horses and they do stupid things, no matter how well trained they are. One thing too, most harness has an over check on it...I use ours but we don't jack the head back like many do, I want my mares to be able to use their head for balance but not drop their head to try to eat grass or fit faddle around. Many people here use a liverpool bit with great success and they aren't considered a harsh bit.
I just cannot stress safety enough and caution. As for her being a witch and you never driving in pairs, don't make excuses for your mare. If you harness her to something, again, she should listen and she should do what you tell her to do. My big Percheron mare, Trixie has a habit of nipping and flipping her ears at Smoke or trying to take a nip out of her. I too made excuses for her, oh she had a bad abscess on her chest and remembers that, oh she may be sore, oh she probably has something pinching, etc...nope. She was being a bitch, plain and simple and a good friend and mentor had to wake me up that she was doing what I was allowing her to get away with. Now, when she acts the fool she gets corrected immediately, be it a sharp word or her name, a jerk on her lead or if it warrants, as she has tried to nip the back of my husbands back, a slap on the neck and/or chest. We do a lot of public education and drives so having our team pay attention is priority. I am fortunate that they are an older team, they both have worked on a professional carriage service and work well together, as they now work mostly off of voice instead of lines.
Most of all, other than safety, have fun.