3. It was always the plan. I do wonder if RTD always wanted a non-white Tardis crew but felt he couldn't jump straight in there so he went with Millie in the first season.
4. It's always possible that Millie decided it was too much for her (yes she was in 200 odd episodes of Corrie but that's much more of an ensemble and I imagine a far less pressured work environment. We need to remember she is still relatively young). This might have coincided with Varada impressing in Boom and the stars aligned.