I think this is a difficult situation because you obviously have a lot of stress related to your job and with you boss. You're scared both of being assualted during pregnancy and of being transferred to a role that you are not familiar with. In addition your recent stress and your pregnancy hormones are magnifying your emotions.
When people outside the situation see what you have written though, it's difficult to see what else your boss could do because:
You believe that you are at risk in your current situation and want an immediate assessment.
She suggested that you would be able to carry on your current duties and you became very distressed. (This does seem unreasonable of her, especially if she said it unsensitively).
She suggested an alternative and you are unhappy with that too.
I understand that if you don't have a good relationship with your boss - especially if her attitude is poor - then this seems personal. But it would be a tricky situation even with a good boss as your job seems incompatible with pregnancy so you would need to change to a less risky role anyway.
Have you explicitly said that you are happy to deal with the less violent people? Is it possible that she isn't disguishing between the violent/less violent people because they could all be a risk to you? Just because someone isn't violent all the time doesn't mean you are 100% safe.
If you were to work in this other department then you might be more safe and have less stress related to your working relationship with your boss. I know the travel could be a hassle and getting used to new collegues and work might be difficult, but it might be the best thing whilst you are pregnant. You might find you like it and want a permanent transfer when you return to work!
I'm sorry, because it doesn't seem like there is an outcome that is 100% brilliant for you at the moment. The important thing is that you and your baby are safe and exposed to as little stress as possible.
Make time to do things you enjoy and that you feel relaxed about so that you and your baby are getting a break from this work related stress. Hopefully someone else will reply with more details on how you can deal with the whole risk assessment/unfriendly boss situation.