pretty much nailed it. oberyn was there for dorne, and for revenge. everything else that factored in to it was either coincidence or a convenient advantage (tyrion being allies with dorne could have been helpful for myrcella's claim to the throne).Zhukov said:In the books he took part in the trial by combat for a chance to kill The Mountain to avenge the deaths of his family members during the rebellion.
He didn't give the slightest of shits for Tyrion's innocence.
you seemed to have missed the meanings behind oberyn's character