Quentyn, quite frankly, was in way over his head. He should have never been sent out on this mission by he's father. I remember in aFfC Arianne outright stated that her father couldn't plot his way out of a cardboard box. So far she's been spot on. Quentyn was essentially sent off as a beggar to confront Daenerys. Not retinue, no ships, no nothing. Just, hello your Grace, here's a contract you never heard of and now you have to marry me. It was destined for failure.

Doran appears to want risk free vengenane. He wants not to be accountable if it fails. He wants 50k Dothraki and/or dragons as back up. He wants it handed to him, as opposed to getting his hands dirty and taking it. He should be praised for keeping Dorne out of a costly war, but beyond that, his plots and schemes have been utter failures. For all his talk of justice and vengeance, he's achieved nothing to accomplish that. Oberyn may be dead, but at least he managed to kill the man who raped and murdered his sister.