If Serena only saw Justine after she initiated the service motion, she could have stopped it before ball contact.
If she already made ball contact before sighting Justine's gesture, it should not have hindered her serve.
Here all I am saying is Justine might have felt Serena ignored her request and tried to score an easy point. Then it is easy to see why Justine felt justified in lying to the referee in order to deny Serena another free serve.
you really haven't seen ever seen players continue to hit their serves once they begin their motion even when their opponent says something/holds up a hand etc? or some other delay? like an umpire saying, 'wait, please?'
I've probably seen that a hundred times over the years. it was perfectly normal what serena did & its pretty bizarre to think that henin thought that serena was trying to get an easy point(that would mean in that brief moment when Serena started her motion, she expected the umpire not to see Henin & was then trying to win the point after she saw that? uh, ok. I'm sure she was expecting the usual, 'wait, please' from the umpire that has happened hundreds of time when a returner asks for time when they aren't ready after a server has started their motion)
it's not easy to stop your serve motion once you start it, and what's the point in not stopping, you know the point is dead once your opponent holds you up anyway. the players that don't hit their serves when that happens often awkwardly stop mid motion/try to catch the ball etc. that can be kind of jarring.
look at edberg continuing to hit his serve when becker audibly says something at '88 Queens(umpire made a major error in not giving Edberg a 1st serve, just another 2nd serve - he then double faulted. and Becker was criticized quite a bit for gamesmanship at the time)