Hi there! Sounds like you've had a rough go of it feeding wise! Hope things continue to get better!
In regards to the milk question, I would be more inclined to keep her on Nutramigen than to use soy milk as her regular milk. Not sure if you are UK based or North America so I suppose that may be a factor as if you are paying for it, it's stupidly expensive. (Oh, wait, just noticed you said soy formula is an option as well...) SO then, either way, I would for the formula (soy or nutramigen) rather than soy milk as her drink. The main reason being that soy milk isn't really fully nutritionally complete - it is quite low in fat compared to milk and it lacks some key vitamins (I *think* that it's Vit B12, and Vit D but can't remember) as well as a couple of other things. While formula, including soy formula, has all of the added vitamins, nutrients and fat content for LO. (Right, as you questioned in your post...maybe I should read more carefully!!!!)
With the introduction of new foods, I went slowly for a long time, probably until about 9ish months. Once I hit that point, I still watched and was careful, but I wasn't so strict about only a new one every three days, etc. I just got a bit more carefree with things, and the few times DS did have a reaction I could limit it to one of a few different foods which I could then go back and try. Rather than when he was very young and I was very strict about the new foods and watching for reactions. When you're already dealing with food allergies it puts you much more on your guard, I have lots of family and friends who rolled their eyes behind my back.
Aside from the milk and soy and sweet potato, does she have any other food allergies? I would look up and read a bit about sweet potato allergy, see if there is any info on what generally the allergic part is, if there are any cross tolerant foods (like, if you are allergic to SP then it's likely you are allergic to xyz, etc) and avoid those things. But, other than that, I would plow on with things. One of the reasons I would push on is because there is sort of a limited time window when LOs are willing to try new things, and if you don't encourage it when she is young, you may very well end up with a fussy eater. Which is okay if you are able to deal with that, but it does make life easier when you have more options to feed her. If she only has probs with diary and sweet pots, I would move forward and just keep an eye on her as you go, watching for any reactions!
Is there any plan for a milk challenge now that she is one? We passed the soy challenge when DS was about 11 months and he got some milk accidentally at 13 months and was fine, so we went on from there. It has taken over a month of weaning his formula into cow's milk, but he's almost there. Other allergies he's also grown out of, and life is so much easier!
However, we have been advised to hold off on shelllfish until 3 and nuts until 5!
Good luck and HTH!