There is a 10 minute window after a defender has been removed where no one can add another defender.
This means that the gym can't be refilled endlessly while someone attacks it...and that the attacker actually has some kind of chance.
Without it, there are areas where it would be pointless to even bother attacking a gym. That would just be counter productive for the game.
"ball sticking with the last ball that was used to catch something" is a new feature so that players with Go+ (or Gotcha) who want to use Great Balls and save Poke Balls for their Go+ can do so without needing to reselect for every single thing they catch.
It is just a change that in a couple of weeks we will all deal with automatically, without thinking about it. It won't take long to adjust.
