When the goddamn king tells you to stop flipping out like a madman anybody who wants to keep their head on their shoulder would stop. That's hardly obeying a command. Prior to this he'd lost his shit and was laying into the Tyrrell's prince with intent to kill, then fought his own brother with intent to kill, all after losing a jousting match and decapitating his own horse. And even after listening to the King (a delayed reaction compared to The Hound) he simply stormed off as a big "fuck you all". He managed Harrenhal in Tywin's absence and spent the entire time having people tortured. And Tywin sent him to burn the countryside when Rob and co decided to start a war.
I don't think The Mountain is so much a strict follower of orders (probably the opposite) as a nutcase with a set of unwritten rules and disclaimers you'll unavoidably get if you use him, and I figure Tywin knows this. He wouldn't need to tell him "sack the castle, kill the queen and her kids". He'd just tell him to take the castle knowing full well what he'd do without uttering the actual words.
Indeed, which is why i think to consider his (partial?) confession a "win" for Oberyn is an incredible stretch.
He didn't gain anything.
The realm of Game of Thrones doesn't really need physical evidence. This is not Law & Order: Westeros. Hearsay and rumours are enough to kill someone. The confession of a loyal soldier combined with the death of Oberyn can be enough for Doran Martell to re-allign his house with the Targaryens.
Which is why, if they already wanted to, they would've.
They don't need a nobody's confession to act on something.
The Mountain is a brainless maniac, his words aren't worth that much to anyone, and most likely everybody already *knew* all they needed to know, so if they wanted or not to go against the Lannister, they could've with or without the Mountain's words.
I think to consider it a winning move, or even a silver lining, to Oberyn's actions, is reaching for the stars and beyond.
So the Mountain killed someone, so what? Tywin can very easily say he didn't order any such thing, and execute the guy (if he isn't already dead), the Martell can team up with the Targaryens (or what's left of them) like they could before, regardless of Mountain's words, which have little to no weight on the whole matter.