Well first it should be noted this is an exhibition match and tennis exhibitions often fuck around and do stupid shit that would never be allowed in professional play like invading your opponent's side of the net or putting more than one ball in play.
The rules are that a server has to stand outside the court and hit the ball so that it bounces into their opponent's court side in the appropriate service box. So there's technically nothing wrong with his serve but he does start bouncing another ball pretending that he hasn't served yet which would count as a deliberate "hinderance" of play as he's using a second ball to distract, I'd say it would count as a service fault and he'd take a second serve if this was a "real" match.
The problem was trying to do "tricky tactics" is that the receiver has to be "ready" to receiver the serve so trying to "trick" them isn't viable as it would require taking the receiver unaware and thus an invalid serve.
On the topic of underhand serving it's rarely that good an idea because, while ok for the very occasional surprise, it gives away all the advantage a server has because the receiver will get to it and return it in play, taking the advantage of hitting the ball from a very deep position in court and thus wins the point when an ATP pro would never want to lose any points on serve especially on their 1st serve.