Do defrosting plates work in reverse

If you're unthawing them in your refrigerator (below 40 F), then you should be just fine. Most beasties don't reproduce at any significant rate below 40 F.

There isn't a clean cut off point because it depends entirely on the existing level of contamination present in your meat.

You should be aware that repeated cycles of thaw/freeze will really damage the texture of your meat. The cells simply do not hold up to that.

Well, you can always add some broth -- chicken would be good. That would allow you to control the salt more so than using boullion crystals.

As far as cooking the vegetables, it depends on whether there are already pieces of vegetables in the frozen soup. If yes, then you may want to cook your add-ins first so that the stuff that's already in the soup doesn't get mushy. If no, you could cook them in the soup.

(However, I have a caveat as far as cooking noodles in broth. When you cook noodles, some of the starch from the pasta transfers into the cooking liquid. This is why pasta water is often used to help with the texture of pasta sauce. If you don't mind having the starch in your soup, you can go ahead and cook the noodles there. If you are bothered by that, cook the noodles separately, drain, and then add them.)