Thats the lore, not actual "fact" if you will. In dawngaurd we physically meet two Snow Elves, one of which clearly states that there was hundreds of them at that location before it was over-run who did not take the dwarves "help". Its not outragious to assume that they are not the only ones who were able to hide away from both factions. We could well find more in a later DLC.
Yes. Plus, regardless of whether or not they accepted the Dwarves' toxins or were betrayed, the Dwemer are dead so they are logically no longer being subjected to such toxins and over generations they may regain sight again. Plus, there are more enclaves of the Snow Elves apparently. I really want some DLC which adds them as a playable race. Of course, it would require a lot of work as Bethesda would have to create an alternate starting area and write a unique campaign for them to avoid clashing with existing lore. It is a lot of work, so I'll doubt they'll do it. But, I can always hope!