2. Once a rally starts, does the server really have much of an advantage or is it equal?
It depends on the ability to return the ball. Solid
server will often have the advantage in the rally due to either speed and placement of the first serve or the placement and topspin of the 2nd. If not by ace, in some of the "easy" points if he finishes off a short returned ball or not is up him.
And it is just hard to consistently return serves firstly because of the speed because your reaction time and focus has to be perfect. Secondly because of the nasty topspin that forces you to stand far behind the baseline in order to retrieve it, or try to get the high ball on the rise. That's why usually great returners are one of the best players in the world imo, because they combine all of these factors (athleticism, focus, reaction time, hand skills)