Are we arguing that a lawyer is bound to present evidence that is adverse to his client? That simply is untrue. The attorney's first and foremost duty is to zealously represent his client. An attorney who presents evidence adverse to the interests of his client would quickly find himself in all sorts of ethical trouble with his local bar association. BTW, the ABA has nothing to do with any of this.
An attorney presents evidence contrary to the interests of his client only when compelled to do so by a court -- this can be in many forms including pre-trial discovery. An attorney cannot lie about the existence of evidence or suborn perjury. There are ethical rules as to what an attorney must do if he realizes that his client is testifying untruthfully, which is why a defense attorney will almost never ask his client is he committed the crime.
The one time when an attorney must voluntarily disclose evidence that is adverse to his client's interests is in a criminal case -- a prosecutor must voluntarily disclose all exculpatory evidence.
As for sports agents and their ethics, please don't make me laugh. They are the scum that scum scrapes off its shoes. If people think they are not leaking information regarding the details of existing bids, you are kidding yourself.