Afaik, Unity damage reduction is applied after all other forms of reduction, reducing the final amount of damage by another 50%. So yes to your first question. As for the 2nd question, the calculation is not correct. Assuming you have 50% elite damage reduction, it won't go up to 100%, since that would mean you don't take damage. Perks of percentage calculation
But the general idea is not wrong, you could say that Unity essentially doubles the effect of all stats providing toughness on your gear. But that is just a more complicated way of saying "Unity doubles your toughness" or "Unity halves incoming damage".