On point 1 I checked the polls and they look no different than Bidens. Recent articles say the same thing. So that can't be why they chose her.
"A flurry of polls conducted in the wake of the June 27 presidential debate showed Harris performing roughly the same as Biden against Trump (who has been leading the president by a slim margin for months), and more recent polls after the attempted assassination of Trump show similar trends."
"A flurry of polls conducted in the wake of the June 27 presidential debate showed Harris performing roughly the same as Biden against Trump (who has been leading the president by a slim margin for months), and more recent polls after the attempted assassination of Trump show similar trends."