Honestly, I think the number of niqab wearing women visiting Disney is SO minuscule, this really is a red herring and not worth getting worked up about.
Here in Canada we had to endure months of debate over whether niqab wearers should be forced to unveil in order to take the citizenship oath. How many women wanted to wear their niqab in a citizenship ceremony? Exactly two. And for those two women our previous government implemented a ban. Which then went to the Supreme court, and was ultimately struck down, wasting tons of taxpayer monies and getting everyone all stirred up about "those people".
It's divisive and not worthy of us.
If, by some chance, a single niqab wearing woman visits with her family, I am quite sure Disney security is capable of making whatever determination they need to make, regarding any possible threat she might pose to the rest of us. If they decide she's allowed to enter the park, that's between her and them. It's none of my business. She deserves just as much of my courtesy and kindness as any other park guest.
Edit: As it happens, on the bus in our hometown, our way to the airport to fly to Disney this past November, my husband and I chatted with a lovely - and very friendly - young lady wearing a niqab. She was a foreign student from Saudi Arabia and happened to be studying at the same university as our daughter. It was interesting - after a few minutes of talk, I realized that I could see more expression and emotion and individuality in her eyes, than I could ever see in a person wearing sunglasses, even if the rest of their face was uncovered.
(My son calls them "authority glasses" when cops wear them.

)