Good on you for trying to find something she can "eat" as such, even if it is just sugary ice or candy floss. Just getting her jaw used to the physical art of chewing is beneficial. Not to mention the social developmental benefits of feeling as ive she is "fitting in" with other kiddies.
I work for a Dentist with over 40 years experience and just asked him about this:
While yes straight sugar isnt going to be great for her teeth, formula is full of sugar too, as is alot of other foods and drinks mums give their bubs without a 2nd thought. 20 months is more than old enough to be encouraging 2x daily teeth cleaning and so as long as you are brushing her teeth they will be fine. You would be amazed at the amount of parents who dont brush their childrens "baby teeth" as they believe they are only going to fall out anyway. As long as you are super strict with brushing (even without toothpaste just incase she cant have that) and follow any high sugar foods or drinks with water when possible she will be fine. Maybe ask the specialist if she could tolerate Fluoride or calcium supplements? They are widely recommended for babies and children at risk of extreme tooth decay.
Goodluck with finding things she can have and I can only hope the severve food allergys decrease as she grows up