Hi hon,
First let me just say that I read your story and I am crying for you right now. I can't even imagine how devastating a late mc of twins must be. I am so, so sorry. I wish I could give you a great big hug.
On the medical negligence front, I think I would discourage that course of action in your case. I have complete trust in your intuition that something was amiss, however the symptoms you describe, cramping and constant bladder pressure, were just daily life during my healthy pregnancy. It sounds as though your doctor was doing fetal monitoring for heartrate, which would be the kind of thing you'd do to keep tabs on fetal health if you were worried.
The tragic fact of the matter is that modern medicine is just not advanced enough to detect all problems in time to save all babies. Twins are particularly vulnerable to loss. My aunt went through the same thing as you with her twins. This happens all the time -- which doesn't make it even a tiny bit easier. It's terrible!
I think that we want so badly to get closure and part of looking for that closure is looking for REASONS for such a terrible loss -- but sometimes there just won't be solid reasons to be found. What I'm trying to say is that I feel rather strongly that, though with a lawyer you might be able to get some sort of penalty placed on your doctor, I don't really think it was her fault, and I also don't think that this kind of blame-placing is particularly constructive to your emotional recovery. It would feel good, in a way, to be able to say, "It was her fault." But sometimes things just aren't so simple.
I think that part of finding closure in a very complex and very emotionally wrought situation like yours is getting past that need for an answer. Acceptance. If you can work through your grief and anger and confusion to a sense of peace (even peace in great sadness), that is going to help you far more than the legal route.
Whatever you choose, I wish you peace and strength and love. Most of all, I wish you healthy happy pregnancies in the future. I wish I could do more!