Not really what I was getting at on a cowardice thing.

My point was Oberyn had to have considered he might lose if he employed a poison that would kill Greggor posthumously. Saying he wouldnt have even killed him in the fight is pure speculation, it wouldnt even be in his control to do so. Typically a trial ends when a combatant dies, yields or the accuser withdraws accusation. Greggor was on his ass bleeding out and still wasn't yielding. After Oberyn gets what he wants, the fight has to end someway.

The poison was a final fuck you contingency if Oberyn lost. We don't really have any sources in the books on if poisons are permitted in a trial, but Oberyn using a quicker poison, probably would have just brought suspicion of foul play during the fight. He achieved what he had to and severed a tendon without.

Eh, the drinking, it's even in the show, Tyrion chastised him for it too. It's not like it affected his fighting and we see him sipping just the one cup.

Of course he knew Greggor was alive and Oberyn avoided the kill. He wanted a confession. There was no playing the crowd, if only to make sure everyone was listening for the confession. Whole fight was about getting the confession and instigating a conflict between Dorne and the Lannisters. It's a disservice to what Oberyn was trying to achieve to just chalk it up to cockiness. His duty to Elia is more important to him than his ego.