What it seems to me is that the legal system has a blind spot: they are very much obsessed with jurisdiction and ethics rules, but don't know what to do when those are in conflict. What happens when you insist that the only venue for a case is one that demands bribes and does not try cases impartially? Chevron seems to think that Ecuadorian corruption meant they were above the law - it's the same logic as the Idaho Zone of Death[1]. Donziger figured that if he's being told to try the case there by an American judge, than it's OK to pay the bribes necessary to get the ruling he wants.
The only reason why that even seems remotely OK is purely a function of how dirty and awful oil companies are. Had this happened to almost anyone else the countersuit on Donziger would have been considered to be a moral imperative. If a lawyer bribed a judge in another country to get a quick judgment on, say, a bakery or something, we'd be up in arms and wanting to see that lawyer behind bars. So perhaps this is a new benchmark for unsympathetic plaintiffs somehow. You are such a bad person that we don't care if you got extorted by a dirty judge-bribing lawyer.
That's, of course, assuming the judge-bribing happened. The person who delivered the testimony has been rather unreliable about this; and has recanted and un-recanted his testimony multiple times. I don't necessarily treat this as evidence that nothing happened and that he made it up. It's entirely possible that this guy just plays up all sides for himself. He could have taken a bribe from Donziger, then "protection from Ecuador" from Chevron, and then more money or favors from the other international tribunals he testified at later. That doesn't exonerate Donziger, it just implicates Chevron, too.
The ultimate take-away here is that the legal system has tunnel vision and is easy to distract. Allegations of unethical conduct by lawyers is enough of an internal threat to the functioning of the courts that they will forget the original crime in favor of circling the wagons. Scumbag lawyers like Chevron's legal team know that they can gin up an ethics trial to get out of their own misconduct.
[0] Or at least, as favorable as you can get for a company that dumped oil on indigenous peoples
[1] A small strip of Yosemite National Park in Idaho where it is constitutionally impossible to empanel a jury for criminal proceedings because Congress demanded it be treated as part of a Wyoming judicial district