1. She doesnt have too. Her fluid nature comes from her absorbong powers and she retains an imprint of people she absorbs. So it is a function of her powers that is canon and I know you agree because I have seen you argue this point before.
2. See above. The personalities were already their due to her previous absorptions. Hence why Sage was able to unlock those powers and allow her to use powers of everyone she had ever absorbed.
3. And yet he is no longer there. Fooling is more complex not better because it requires Gambit to make Bogan think he has possession. So Gambit had to mimic being possessed successful enough that Bogan couldnt tell he was in control.